8. DRUGS ACTING ON CENTRAL NERVOUS SYSTEM

DRUGS ACTING ON CENTRAL NERVOUS SYSTEM: ANAESTHETICS, SEDATIVES, HYPNOTICS, ANTIPSYCHOTICS, ANTICONVULSANTS & ANTIDEPRESSANTS: A TEACHER’S COMPREHENSIVE GUIDE

Welcome, future pharmacists and healthcare professionals!

The Central Nervous System (CNS) is the most complex and vital system in the human body, controlling everything from consciousness and memory to movement and emotion. Drugs acting on the CNS are among the most powerful and widely used medications in clinical practice—from anaesthetics that make surgery possible to antidepressants that restore mental health. As a pharmaceutical chemistry educator with years of experience teaching CNS pharmacology, I have observed that students often find this topic challenging due to the sheer number of drugs and their mechanisms. Let me tell you: Understanding CNS drugs is essential for every pharmacy professional.

In this comprehensive guide, I will walk you through the major classes of CNS drugs—general and local anaesthetics, sedatives and hypnotics, antipsychotics, anticonvulsants, and antidepressants. I will explain their classifications, mechanisms of action, uses, stability, and formulations. By the end of this article, you will have a thorough understanding of these essential pharmaceutical agents. Let us begin our journey into the fascinating world of CNS pharmacology!

Dpharmguru’s exam insights:

CNS drugs are frequently tested in pharmacy exams. Remember: Anaesthetics produce loss of sensation, sedatives reduce excitement, hypnotics induce sleep, antipsychotics treat schizophrenia, anticonvulsants control seizures, and antidepressants treat depression. Pay special attention to the mechanisms of action and classifications—these are almost always asked in exams! Also, remember the four stages of anaesthesia—this is a classic exam question.

1. ANAESTHETICS

Anaesthesia is a state of insensitivity induced by an anaesthetic agent that acts on the brain or peripheral nervous system to cause local or general loss of sensation, including pain.

Types of Anaesthetics

• Local Anaesthetics: These drugs reduce pain in particular areas of the body—generally used in dental procedures, minor surgeries, and injections.
• General Anaesthetics: These drugs are used in major surgeries, childbirth, and other procedures that require complete anaesthesia. They produce a loss of consciousness and sensation throughout the body.

Stages of Anaesthesia

• Stage I (Analgesia): This stage begins with the inhalation of anaesthetic gas and ends with the beginning of loss of consciousness.
• Stage II (Delirium or Excitement): This stage starts with the loss of consciousness. The inhibition of higher centres is removed and subconscious emotions become active.
• Stage III (Surgical Anaesthetics): This stage involves unconsciousness and paralysis of reflexes. The patient shows complete tranquillity, pulse goes slower, respiration is full and regular, and pupils become constricted. All surgical procedures are carried out in this stage.
• Stage IV (Medullary Paralysis): This stage should be avoided. It starts with respiratory paralysis and ends with cardiac failure and death.

Classification of General Anaesthetics

• Inhalational Anaesthetics: Volatile liquids or gases delivered using an anaesthetic machine. Examples: Enflurane, Desflurane, Halothane, Ether.
• Intravenous Anaesthetics: Produce anaesthetic effect when injected into the bloodstream. Examples: Thiopentone sodium, Etomidate, Ketamine, Fentanyl, Propofol, Diazepam.
• Gaseous Anaesthetics: Nitrous oxide.

1. Thiopental Sodium

Chemical Name: Sodium 5-ethyl-5-(1-methylbutyl)-2-thiobarbiturate

Mechanism of Action: Thiopental sodium increases the opening duration of Cl⁻ ionophore by binding at a distinct binding site related to the Cl⁻ ionophore at the GABAₐ receptor. Therefore, it prolongs the post-synaptic inhibitory effect of GABA in the thalamus.

Uses:

• Used as a sole anaesthetic agent for short procedures (15 minutes)
• For inducing anaesthesia prior to administering other anaesthetic agents
• For supplementing regional anaesthesia
• For controlling convulsive states
• In neurosurgical patients having increased intracranial pressure
• For narcoanalysis and narcosynthesis in psychiatric disorders

Stability & Storage: Store at 22°C. Remains stable and sterile for 6 days and well beyond 7 days at 3°C.

Formulations: Injection, Powder, Solution

Brand Names: Pentothal, Anesthal, Thiojex, Thiosol, Pentone

2. Ketamine Hydrochloride

Chemical Name: 2-(2-chlorophenyl)-2-(methylamino)cyclohexan-1-one

Mechanism of Action: Ketamine interacts with NMDA receptors by binding non-competitively to the phencyclidine recognition site. It also interacts with opioid receptors, muscarinic receptors, and nicotinic acetylcholine receptors.

Uses:

• Anaesthetic agent in various diagnostic and surgical procedures
• Combined with a muscle relaxant if skeletal muscle relaxation is needed
• Combined with a visceral pain desensitising agent for visceral pain procedures
• For inducing anaesthesia before using other general anaesthetics

Stability & Storage: Keep in tightly closed container in dry and well-ventilated place. Store at 2-8°C.

Formulation: Liquid solution

Brand Names: Ketalar, Anket, Ketmin, Verket

3. Propofol

Chemical Name: 2,6-diisopropylphenol

Mechanism of Action: Propofol depresses the global CNS and directly activates GABAₐ receptors, thereby inhibiting the NMDA receptor and modulating calcium influx through slow calcium-ion channels. It has a short half-life with duration of action of 2 to 10 minutes.

Uses:

• Slows down the activity of brain and nervous system
• Acts as sedative and keeps patient asleep during general anaesthesia for surgery
• Used in adults as well as children 2 months and older
• Used to sedate critical care patients needing a mechanical ventilator

Stability & Storage: Store between 4°C to 22°C (40°F to 72°F). Refrigeration is not required.

Formulations: Emulsion, Suspension, Injectable

Brand Names: Diprivan, Propoven

Dpharmguru’s exam insights:

General anaesthetics are frequently tested. Remember the four stages of anaesthesia—Analgesia, Delirium/Excitement, Surgical Anaesthesia, and Medullary Paralysis. Surgical procedures are carried out in Stage III. Thiopental is ultra-short acting, Ketamine is an NMDA receptor antagonist, and Propofol has minimal respiratory depression. These are classic exam questions!

2. SEDATIVES AND HYPNOTICS

Sedatives reduce excitement and are commonly used as anxiolytics. Hypnotics produce sleep, which resembles natural sleep. Sedative-hypnotic drugs reduce tension and anxiety, and induce calmness (sedative effect) or sleep (hypnotic effect). Low doses exert a calming effect and higher doses have a sleep-inducing effect.

Classification of Sedatives and Hypnotics

• Urea Derivatives:
  • Diureides (Barbiturates):
    • Long Acting: Phenobarbitone, Mephobarbitone
    • Short Acting: Butobarbitone, Secobarbitone, Pentobarbitone
    • Ultra Short Acting: Thiopentone, Hexobarbitone
  • Related Ureides: Glutethimide
• Benzodiazepines: Diazepam, Clonazepam, Alprazolam, Flurazepam, Nitrazepam, Chlordiazepoxide, Lorazepam, Oxazepam
• Newer Non-Benzodiazepine Hypnotics: Zopiclone, Zolpidem
• Miscellaneous: Chloral hydrates, Triclophos, Paraldehyde

1. Diazepam

Chemical Name: 7-chloro-1,3-dihydro-1-methyl-5-phenyl-2H-1,4-benzodiazepin-2-one

Mechanism of Action: Diazepam binds non-specifically to BDZ (benzodiazepine) receptors and induces sleep, anticonvulsant activity, and affects muscle relaxation, motor coordination, and memory. The BDZ receptors couple with GABAₐ receptors and increase GABA affinity, enhancing the effects of GABA.

Uses:

• Treating severe anxiety disorders
• Alcohol withdrawal syndrome
• Managing insomnia for short-term
• Pre-medicant, anticonvulsant, and sedative

Stability & Storage: Diazepam injection is chemically stable for 90 days when stored at 4°C or 30°C. Store in refrigerator.

Formulations: Tablet, Oral solution, Rectal gel, Injectable solution

Brand Names: Valium, Diastat, Diastat AcuDial

2. Alprazolam

Chemical Name: 8-Chloro-1-methyl-6-phenyl-4H-s-triazolo[4,3-a][1,4]benzodiazepine

Mechanism of Action: Alprazolam non-specifically binds with benzodiazepine receptors (BNZ1) and mediates sleep. It also binds to BNZ2 and affects muscle relaxation, anticonvulsant activity, motor coordination, and memory.

Uses:

• Managing anxiety disorder
• Short-term treatment of anxiety symptoms
• Treating panic disorders with or without agoraphobia

Stability & Storage: Store in a dry place at room temperature (68-77°F / 20-25°C). Do not store in bathroom (moisture and humidity can cause breakdown).

Formulations: Tablet, Extended release

Brand Names: Xanax, Xanax XR, Alprazolam Intensol, Niravam

3. Nitrazepam

Chemical Name: 1,3-dihydro-7-nitro-5-phenyl-2H-1,4-benzodiazepin-2-one

Mechanism of Action: Nitrazepam affects central benzodiazepine receptors associated with inhibitory GABA receptors, leading to the enhancement of GABA binding activity.

Uses:

• Panic disorders
• Severe anxiety
• Insomnia
• Seizures

Stability & Storage: Store at room temperature between 59-86°F (15-30°C) away from heat and light.

Formulation: Tablets

Brand Names: Mogadon, Nitrazadon, Hypnotex, Nitavan

4. Phenobarbital

Chemical Name: 5-ethyl-5-phenyl-2,4,6(1H,3H,5H)-pyrimidinetrione

Mechanism of Action: Phenobarbital increases synaptic inhibition by acting on GABAₐ receptors. This elevates the seizure threshold and reduces the spread of seizure activity. It may decrease excitatory transmitter release by inhibiting calcium channels.

Uses:

• Hypnotic, sedative, and antiepileptic drug
• Symptomatic therapy of epilepsy
• Nervous and tension-related states

Stability & Storage: Store in well-closed containers at controlled room temperature 15-30°C (59-86°F).

Formulations: Oral elixir, Oral tablet

Brand Names: Luminal, Solfoton

3. ANTIPSYCHOTICS

Anti-psychotic drugs are mainly used for treating schizophrenia; however, they can also be used in mania with much agitation. They are also called mood-altering agents.

Classification of Antipsychotics

• Phenothiazine Derivatives: Chlorpromazine, Trifluoperazine
• Butyrophenones: Haloperidol, Trifluperidol
• Rauwolfia Alkaloids: Reserpine
• Diphenylbutyl Piperidines: Pimozide, Penfluridol
• Atypical Neuroleptics: Clozapine, Olanzapine, Risperidone
• Indole Derivatives: Malingole
• Thioxanthine Derivatives: Chlorprothixene, Flupenthixol
• Substituted Benzamides: Sulpirides

1. Chlorpromazine Hydrochloride

Chemical Name: 3-(2-chlorophenothiazin-10-yl)-N,N-dimethylpropan-1-amine

Mechanism of Action: Chlorpromazine acts by blocking post-synaptic D₂ dopamine receptors. Its antipsychotic effect is due to dopamine receptor blockade in the mesolimbic area, and blockade in the nigrostriatal system produces extrapyramidal effects.

Uses:

• Treating schizophrenia
• Controlling nausea and vomiting
• Relieving pre-operative restlessness and apprehension
• Adjunct for treating tetanus
• Relieving intractable hiccups
• Treating severe behavioural problems in children

Stability & Storage: Store at temperature less than 40°C, ideally between 15-30°C. Avoid freezing of oral solutions and injection.

Formulations: Injectable solution, Oral tablet

Brand Names: Ormazine, Thorazine, Thorazine Spansule

2. Haloperidol

Chemical Name: [4-(4-chlorophenyl)-1-[4-(4-fluorophenyl)-4-oxobutyl]piperidin-4-yl]decanoate

Mechanism of Action: It interrupts the impulse between diencephalon and cortex by inhibiting the ascending reticular system of the brain stem. It may also antagonise the actions of glutamic acid in the extrapyramidal system.

Uses:

• Acute psychosis (drug psychosis)
• Hyperactivity, aggression, acute delirium
• Adjunct in treatment of alcohol and opioid withdrawal
• Severe chronic pain (with analgesics)
• Intractable hiccups

Stability & Storage: Store at 20-25°C (68-77°F). Protect from light. Do not refrigerate.

Formulations: Tablets, Oral Concentration

Brand Names: Haldol, Haldol Decanoate, Haloperidol LA, Peridol

3. Risperidone

Chemical Name: 3-{2-[4-(6-fluoro-1,2-benzoxazol-3-yl)piperidin-1-yl]ethyl}-2-methyl-4H,6H,7H,8H,9H-pyrido[1,2-a]pyrimidin-4-one

Mechanism of Action: Risperidone inhibits serotonergic 5-HT₂ₐ and dopaminergic D₂ receptors in the brain. It binds with high affinity to 5-HT₂ₐ, approximately 10-20 times greater affinity than binding to D₂ receptors.

Uses:

• Treating schizophrenia
• Mood disorders like bipolar disorder
• Depression with psychosis

Stability & Storage: Store at room temperature away from light and moisture. Liquid form should not be refrigerated.

Formulations: Tablets, Oral Suspension

Brand Names: Perseris, Risperdal

4. Sulpiride

Chemical Name: N-[(1-ethylpyrrolidin-2-yl)methyl]-2-methoxy-5-sulfamoylbenzamide

Mechanism of Action: Sulpiride is a dopamine D₂ receptor antagonist. It lacks effects on norepinephrine, serotonin, histamine, acetylcholine, or GABA receptors.

Uses:

• Treating schizophrenia
• Digestive aid
• Antipsychotic and antidepressant

Stability & Storage: Store in a cool and dry place away from direct heat and light.

Formulations: Tablets, Capsules, Solutions

Brand Names: Dogmatil, Dolmatil, Eglonyl, Espiride, Modal, Prometar, Sulpor

5. Olanzapine

Chemical Name: 2-methyl-4-(4-methyl-1-piperazinyl)-10H-thieno[2,3-b][1,5]benzodiazepine

Mechanism of Action: Antagonism of multiple neuronal receptors including dopamine receptors D1, D2, D3, D4; serotonin receptors 5HT2A, 5HT2C, 5HT3, 5HT6; alpha-1 adrenergic receptor; histamine receptor H1; and multiple muscarinic receptors.

Uses:

• Treating psychotic conditions such as schizophrenia
• Bipolar disorder or manic depression (adults and children at least 13 years old)

Stability & Storage: Store at room temperature between 68-77°F (20-25°C). Keep away from light. Do not store in moist or damp areas.

Formulation: Tablets

Brand Names: ZyPREXA, ZyPREXA Zydis

6. Quetiapine

Chemical Name: 2-(2-(4-dibenzo[b,f][1,4]thiazepine-11-yl-1-piperazinyl)ethoxy)ethanol

Mechanism of Action: Antagonism of dopamine type 2 (D2) and serotonin 2A (5HT2A) receptors. Antagonism of H1 receptors, adrenergic α1 receptors, and muscarinic M1 receptors leads to somnolence, orthostatic hypotension, and anticholinergic effects.

Uses:

• Treating schizophrenia (adults and children at least 13 years old)
• Treating bipolar disorder (adults and children at least 10 years old)
• Together with antidepressant medications for major depressive disorder

Stability & Storage: Store at room temperature between 59-86°F (15-30°C). Keep away from light. Do not store in moist or damp areas.

Formulations: Tablets, Capsules

Brand Names: SEROquel, SEROquel XR

7. Lurasidone

Chemical Name: (1S,2R,6S,7R)-4-[[(1R,2R)-2-[[4-(1,2-benzothiazol-3-yl)piperazin-1-yl]methyl]cyclohexyl]methyl]-4-azatricyclo[5.2.1.02,6]decane-3,5-dione

Mechanism of Action: An atypical antipsychotic that is a D2 and 5-HT2A antagonist (mixed serotonin and dopamine activity) to improve cognition.

Uses:

• Schizophrenia in adults and adolescents
• Depressive episode associated with Bipolar I Disorder

Stability & Storage: Store at room temperature away from light and moisture.

Formulation: Tablets

Brand Name: Latuda

4. ANTICONVULSANTS

Anticonvulsants are also known as antiepileptic or anti-seizure drugs. They are used for adequately controlling and managing CNS disorders manifested by recurrent transient attacks of disturbed brain function, producing motor (convulsive), sensory (seizure), and psychic sequence of events.

Classification of Anticonvulsants

• Barbiturates: Phenobarbital, Mephobarbital, Metharbital
• Hydantoin Derivatives: Phenytoin, Mephenytoin, Ethotoin
• Oxazolidinedione Derivatives: Trimethadione, Paramethadione
• Succinimides: Phensuximide, Methsuximide, Ethosuximide
• Benzodiazepines: Clobazam, Clonazepam, Diazepam, Clorazepate
• GABA Analogues: Progabide, Tiagabine, Vigabatrin, Gabapentin
• Miscellaneous: Carbamazepine, Valproate, Phenacemide, Primidone
• Newer Anticonvulsants: Lamotrigine, Topiramate, Zonisamide, Fosphenytoin

1. Phenytoin

Chemical Name: Sodium 5,5-diphenyl-2,4-imidazolidinedione

Mechanism of Action: Phenytoin limits the spreading of seizure activity and reduces seizure propagation by acting on the sodium channels present on the neural cell membrane. It promotes sodium efflux from neurons, reducing post-tetanic potentiation at synapses.

Uses:

• Status Epilepticus: Administered by slow intravenous injection
• Trigeminal Neuralgia: Second choice drug to carbamazepine
• Cardiac Arrhythmias: Especially digitalis induced
• Generalised tonic, clonic, simple and complex partial seizures

Stability & Storage: Store in tightly closed or sealed container.

Formulations: Capsule, Tablet, Oral suspension, Injectable solution

Brand Names: Dilantin, Phenytoin Sodium, Prompt

2. Carbamazepine

Chemical Name: 5H-dibenzo[b,f]azepine-5-carboxamide

Mechanism of Action: Continuous repetitive firing is inhibited by carbamazepine by blocking the use-dependent sodium channels. Pain relief is associated with blockade of synaptic transmission in the trigeminal nucleus.

Uses:

• Treating epilepsy
• Pain related to trigeminal neuralgia
• Bipolar disorder

Stability & Storage: Store in a cool, dry place at room temperature. Keep away from direct sunlight and children.

Formulations: Tablet, Suspension, Capsule

Brand Names: Tegretol, Equetro, Epitol, Tegretol XR

3. Clonazepam

Chemical Name: 5-(2-chlorophenyl)-1,3-dihydro-7-nitro-2H-1,4-benzodiazepine-2-one

Mechanism of Action: The effects of GABA are potentiated by allosteric interactions between central BDZ and GABA receptors. This results in increased inhibition of the ascending reticular activating system.

Uses:

• Myotonic or atonic seizures
• Nocturnal myoclonus
• Photosensitive epilepsy
• Absence seizures
• Lennox-Gastaut syndrome
• Panic disorders

Stability & Storage: Store at 59-86°F (15-30°C). Do not store in moist or damp areas.

Formulations: Tablets, Solution

Brand Names: Klonopin, Rivotril

4. Valproic Acid

Chemical Name: 2-propylpentanoic acid

Mechanism of Action: Valproic acid dissociates into valproate ions, which binds to and inhibits GABA transaminase. Its anticonvulsant activity is due to increased concentrations of GABA in the brain. It may also suppress repetitive neuronal firing by inhibiting voltage-sensitive sodium channels.

Uses:

• Managing and treating seizure disorders
• Epilepsy
• Mania
• Prophylactic treatment of migraine headache
• Absence seizures, tonic-clonic seizures, complex partial seizures

Stability & Storage: Store between 15-30°C (59-86°F) in a tight container. Protect from freezing.

Formulations: Capsule, Syrup, Injectable solution

Brand Names: Valproate Sodium, Depakene, Depacon, Stavzor

5. Gabapentin

Chemical Name: 1-(aminomethyl)cyclohexaneacetic acid

Mechanism of Action: Gabapentin interacts with cortical neurons at auxiliary sub-units of voltage-sensitive calcium channels. It increases the synaptic concentration of GABA and reduces the release of mono-amine neurotransmitters.

Uses:

• Managing postherpetic neuralgia in adults
• Adjunctive therapy for partial seizures with and without secondary generalisation

Stability & Storage: Stable at least 56 days at 25°C and at least 90 days when refrigerated or at room temperature.

Formulations: Capsule, Tablet, Solution, Suspension

Brand Names: Neurontin, Gralise, Gabarone, Fanatrex

6. Topiramate

Chemical Name: 2,3:4,5-di-O-isopropylidene-beta-D-fructopyranose

Mechanism of Action: GABA-A receptor activity is stimulated by topiramate at brain non-benzodiazepine receptor sites and reduces glutamate activity at both AMPA and kainate receptors. It also blocks voltage-dependent sodium channels.

Uses:

• Treating certain types of seizures
• Preventing migraine headaches

Stability & Storage: Tablets store at room temperature away from excess heat and moisture. Sprinkle capsules store at or below 77°F (25°C).

Formulations: Tablets, Capsules

Brand Names: Topamax, Trokendi XR, Topamax Sprinkle, Qudexy XR

7. Vigabatrin

Chemical Name: 4-amino-5-hexenoic acid

Mechanism of Action: The brain concentration of GABA is increased by vigabatrin through irreversibly inhibiting enzymes that catabolise GABA (GABA transaminase, GABA-T).

Uses:

• Adjunct therapy for refractory complex partial seizures
• Monotherapy for infantile spasms in infants 1 month to 2 years

Stability & Storage: Store at 20-25°C. Powders store at room temperature away from light and moisture.

Formulations: Tablets, Powders

Brand Names: Sabril, Vigadrone

8. Lamotrigine

Chemical Name: 6-(2,3-dichlorophenyl)-1,2,4-triazine-3,5-diamine

Mechanism of Action: Lamotrigine inhibits voltage-sensitive sodium channels, stabilising neuronal membranes, thereby modulating the release of presynaptic excitatory neurotransmitters. It suppresses the release of the excitatory amino acid, glutamate.

Uses:

• Alone or with other medications for epileptic seizures in adults and children
• Delay mood episodes in adults with bipolar disorder

Stability & Storage: Store in tightly closed or sealed containers.

Formulation: Tablets

Brand Name: Lamictal

5. ANTIDEPRESSANTS

Antidepressants are mood-elevating and anti-agitational drugs used to rehabilitate mentally depressed patients for improving their mental status. Two main types of depression are recognised:

• Unipolar Depression: Occurs earlier in life and tends to be inherited. More common than bipolar depression.
• Bipolar Depression: Mood and behaviour fluctuate between depression and mania.

Classification of Antidepressants

• Tricyclic Antidepressants (TCAs): Imipramine, Amitriptyline
• Monoamine Oxidase Inhibitors (MAOIs): Phenelzine
• Atypical Antidepressants: Maprotiline, Trazodone, Fluoxetine
• Selective Serotonin Reuptake Inhibitors (SSRIs): Fluoxetine, Sertraline, Citalopram, Fluvoxamine, Escitalopram, Paroxetine
• Serotonin-Norepinephrine Reuptake Inhibitors (SNRIs): Venlafaxine, Duloxetine

1. Amitriptyline Hydrochloride

Chemical Name: N,N-dimethyl-3-(2-tricyclo[9.4.0.03,8]pentadeca-1(15),3,5,7,11,13-hexaenylidene)propan-1-amine;hydrochloride

Mechanism of Action: Amitriptyline gets metabolised to nortriptyline which inhibits the reuptake of norepinephrine and serotonin almost equally. The membrane pump mechanism responsible for uptake of norepinephrine and serotonin is inhibited.

Uses:

• Widely used in anxiety accompanying depression
• Improves mood and feeling of well-being
• Relieves stress and enhances energy level

Stability & Storage: Store in well-closed container at 20-25°C (68-77°F). Protect from light.

Formulations: Tablets, Oral solutions

Brand Names: Elavil, Tryptizol, Laroxyl, Sarotex

2. Imipramine Hydrochloride

Chemical Name: 3-(10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl)-N,N-dimethylpropan-1-amine

Mechanism of Action: Imipramine inhibits the neuronal re-uptake of norepinephrine and serotonin. It binds the sodium-dependent serotonin transporter and sodium-dependent norepinephrine transporter.

Uses:

• Treatment of endogenous depression
• Treatment of nighttime bed-wetting (enuresis) in children

Stability & Storage: Store in tightly closed container in well-ventilated place.

Formulations: Capsule, Tablet

Brand Name: Tofranil

3. Fluoxetine

Chemical Name: N-methyl-3-phenyl-3-[4-(trifluoromethyl)phenoxy]propan-1-amine

Mechanism of Action: Fluoxetine is a Selective Serotonin Reuptake Inhibitor (SSRI) which blocks the reuptake of serotonin at the serotonin reuptake pump of the neuronal membrane. It is metabolised to norfluoxetine.

Uses:

• Treatment of endogenous depression
• Obsessive-compulsive disorder
• Obesity and alcoholism
• Most commonly used antidepressant due to high patient acceptance and lack of lethal side effects

Stability & Storage: Store at room temperature between 59-86°F (15-30°C). Keep away from light. Keep bottle tightly closed.

Formulations: Capsule, Tablet, Solution

Brand Names: Act Fluoxetine, Prozac, Sarafem, Symbyax

4. Venlafaxine

Chemical Name: (RS)-1-[2-dimethylamino-1-(4-methoxyphenyl)-ethyl]cyclohexanol

Mechanism of Action: Venlafaxine and its active metabolite O-desmethylvenlafaxine (ODV) inhibit the reuptake of both serotonin and norepinephrine. They have weak inhibitory effects on dopamine reuptake.

Uses:

• Major depressive disorder
• Anxiety
• Panic disorder

Stability & Storage: Keep away from direct sunlight and children. Store in a cool, dry place.

Formulations: Capsule, Tablet

Brand Name: Effexor

5. Duloxetine

Chemical Name: (3S)-N-methyl-3-(naphthalen-1-yloxy)-3-(thiophen-2-yl)propan-1-amine

Mechanism of Action: Duloxetine is a potent inhibitor of neuronal serotonin and norepinephrine reuptake and a less potent inhibitor of dopamine reuptake.

Uses:

• Major depressive disorder in adults
• General anxiety disorder
• Nerve pain (diabetic neuropathy)
• Fibromyalgia

Stability & Storage: Keep container tightly closed in dry and well-ventilated place. Recommended storage temperature is -20°C.

Formulations: Capsules, Tablets

Brand Names: Cymbalta, Drizalma Sprinkle, Irenka

6. Sertraline

Chemical Name: (1S,4S)-4-(3,4-dichlorophenyl)-N-methyl-1,2,3,4-tetrahydronaphthalen-1-amine

Mechanism of Action: Sertraline inhibits the reuptake of serotonin at the presynaptic neuronal membrane, increasing serotonergic activity.

Uses:

• Major depressive disorder
• Obsessive-Compulsive Disorder (OCD)
• Panic disorder
• Social Anxiety Disorder (SAD)
• Post-Traumatic Stress Disorder (PTSD)
• Premenstrual dysphoric disorder

Stability & Storage: Store at 20-25°C (68-77°F).

Formulations: Tablets, Oral concentration

Brand Name: Zoloft

7. Citalopram

Chemical Name: 1-[3-(dimethylamino)propyl]-1-(4-fluorophenyl)-1,3-dihydro-2-benzofuran-5-carbonitrile

Mechanism of Action: Citalopram inhibits CNS neuronal reuptake of serotonin via the serotonin transporter (SLC6A4).

Uses: Most commonly used to treat depression.

Stability & Storage: Store at room temperature (77°F / 25°C). Keep away from high temperatures.

Formulations: Tablets, Oral Solutions

Brand Name: CeleXA

8. Fluvoxamine

Chemical Name: O-(2-aminoethyl)-N-{(1E)-5-methoxy-1-[4-(trifluoromethyl)phenyl]pentylidene}hydroxylamine

Mechanism of Action: Fluvoxamine inhibits CNS neuronal uptake of serotonin. It has virtually no affinity for adrenergic, muscarinic, dopamine, or histamine receptors.

Uses: Used to treat symptoms of Obsessive Compulsive Disorder (OCD) in adults and children at least 8 years old.

Stability & Storage: Store at room temperature between 59-86°F (15-30°C). Keep away from light.

Formulations: Capsules, Tablets

Brand Name: Luvox

9. Escitalopram

Chemical Name: 1-[3-(dimethylamino)propyl]-1-(4-fluorophenyl)-1,3-dihydro-2-benzofuran-5-carbonitrile

Mechanism of Action: Escitalopram binds to the primary binding site on the serotonin transporter (SERT), preventing the re-uptake of serotonin into the presynaptic neuron.

Uses:

• Major Depressive Disorder (MDD)
• Generalised Anxiety Disorder (GAD)
• Obsessive-Compulsive Disorder (OCD)

Stability & Storage: Store in a closed container at room temperature.

Formulation: Tablets

Brand Names: Cipralex, Lexapro

10. Paroxetine

Chemical Name: (3S,4R)-3-[(1,3-benzodioxol-5-yloxy)methyl]-4-(4-fluorophenyl)piperidine

Mechanism of Action: Paroxetine enhances serotonergic activity via the inhibition of presynaptic reuptake of serotonin by the serotonin (SERT) receptor. It is a stronger inhibitor of serotonin reuptake than citalopram, fluoxetine, and fluvoxamine.

Uses:

• Depression, including major depressive disorder
• Panic disorder
• Obsessive-Compulsive Disorder (OCD)
• Anxiety disorders
• Post-Traumatic Stress Disorder (PTSD)
• Premenstrual dysphoric disorder (PMDD)
• Hot flashes related to menopause (Brisdelle brand)

Stability & Storage: Tablets store at room temperature (59-86°F / 15-30°C). Suspension and controlled release tablets store at or below 77°F (25°C).

Formulation: Tablets

Brand Names: Paxil, Brisdelle, Paxil CR, Pexeva

Dpharmguru’s exam insights:

Antidepressants are frequently tested. Remember: TCAs (Amitriptyline, Imipramine) block norepinephrine and serotonin reuptake. SSRIs (Fluoxetine, Sertraline, Citalopram, Fluvoxamine, Escitalopram, Paroxetine) selectively block serotonin reuptake. SNRIs (Venlafaxine, Duloxetine) block both serotonin and norepinephrine reuptake. A common exam question is: “Which antidepressant is the most commonly prescribed SSRI?” (Answer: Fluoxetine).

COMPARISON: CNS DRUG CLASSES

Drug Class	Primary Action	Key Examples	Mechanism
General Anaesthetics	Loss of consciousness	Thiopental, Ketamine, Propofol	CNS depression
Sedatives/Hypnotics	Calming/Sleep induction	Diazepam, Alprazolam, Phenobarbital	GABA potentiation
Antipsychotics	Treat schizophrenia	Chlorpromazine, Haloperidol, Risperidone	Dopamine blockade
Anticonvulsants	Control seizures	Phenytoin, Carbamazepine, Valproic Acid	Sodium channel blockade
Antidepressants	Treat depression	Fluoxetine, Amitriptyline, Venlafaxine	Monoamine reuptake inhibition

FREQUENTLY ASKED QUESTIONS (FAQs)

1. What are the four stages of anaesthesia?

The four stages are: Stage I (Analgesia), Stage II (Delirium/Excitement), Stage III (Surgical Anaesthesia), and Stage IV (Medullary Paralysis). Surgical procedures are carried out in Stage III.

2. What is the difference between a sedative and a hypnotic?

A sedative reduces excitement and anxiety (calming effect), while a hypnotic induces sleep. Low doses of these drugs exert a calming effect, while higher doses have a sleep-inducing effect.

3. What is the mechanism of action of antipsychotics?

Antipsychotics primarily act by blocking dopamine receptors (D2) in the brain. This reduces dopamine activity in the mesolimbic pathway, which is responsible for the antipsychotic effect.

4. What is the difference between SSRIs and SNRIs?

SSRIs (Selective Serotonin Reuptake Inhibitors) selectively block the reuptake of serotonin. SNRIs (Serotonin-Norepinephrine Reuptake Inhibitors) block the reuptake of both serotonin and norepinephrine.

5. What is the mechanism of action of phenytoin?

Phenytoin limits the spreading of seizure activity by acting on sodium channels on the neural cell membrane. It promotes sodium efflux from neurons, reducing post-tetanic potentiation at synapses.

6. Why is propofol preferred for short procedures?

Propofol has a short half-life with a duration of action of 2 to 10 minutes and minimal respiratory depression. This makes it ideal for short surgical procedures and for sedation in critical care.

SUMMARY

Drugs acting on the Central Nervous System are among the most powerful and important medications in clinical practice. This guide covered the major classes:

• General Anaesthetics (Thiopental, Ketamine, Propofol) produce reversible loss of consciousness for surgery
• Sedatives and Hypnotics (Diazepam, Alprazolam, Phenobarbital) reduce anxiety and induce sleep
• Antipsychotics (Chlorpromazine, Haloperidol, Risperidone) treat schizophrenia and other psychoses
• Anticonvulsants (Phenytoin, Carbamazepine, Valproic Acid) control seizures and epilepsy
• Antidepressants (Fluoxetine, Amitriptyline, Venlafaxine) treat depression and mood disorders

As I always tell my students: “Understanding CNS drugs is essential for safe and effective patient care. These drugs have the power to save lives, restore mental health, and improve quality of life—but they must be used with precision and care.”

REFERENCES AND FURTHER READING

• Pharmacy Council of India (PCI). (2022). Pharmacology Syllabus. New Delhi: PCI.
• Rang, H. P., & Dale, M. M. (2021). Rang & Dale’s Pharmacology (9th ed.). Elsevier.
• Goodman, L. S., & Gilman, A. (2018). Goodman & Gilman’s The Pharmacological Basis of Therapeutics (13th ed.). McGraw-Hill.
• Katzung, B. G. (2021). Basic and Clinical Pharmacology (15th ed.). McGraw-Hill.
• Williams, D. A., & Lemke, T. L. (2019). Foye’s Principles of Medicinal Chemistry (8th ed.). Wolters Kluwer.

Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns. Pharmaceutical regulations and guidelines may vary by region—always refer to your local regulatory authorities for specific requirements.

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