Mechanism of action
Chlorpromazine is a phenothiazine derivative that acts as a non-selective antagonist at multiple central and peripheral receptors. Its antipsychotic effect is primarily mediated through dopamine D2 receptor blockade in the mesolimbic and mesocortical pathways, reducing positive symptoms of psychosis such as hallucinations and delusions.
As a low-potency antipsychotic, chlorpromazine has relatively weak D2 affinity and therefore requires higher doses compared to high-potency agents like haloperidol. This low potency results in significant binding to other receptors:
- Histamine H1 receptors: sedation and weight gain
- Muscarinic acetylcholine receptors: anticholinergic effects (dry mouth, constipation, urinary retention, blurred vision)
- Alpha-1 adrenergic receptors: orthostatic hypotension and reflex tachycardia
- Dopamine D2 receptors in the chemoreceptor trigger zone (CTZ): antiemetic effects
The blockade of dopamine receptors in the nigrostriatal pathway causes extrapyramidal symptoms (EPS), while blockade in the tuberoinfundibular pathway leads to hyperprolactinemia. Chlorpromazine also has membrane-stabilizing properties and weak local anesthetic effects.
- Chlorpromazine is a low-potency first-generation antipsychotic with high anticholinergic and antihistaminic side effects.
- Mechanism: dopamine D2 receptor antagonist in mesolimbic pathway; also blocks H1, muscarinic, and alpha-1 receptors.
- Low-potency antipsychotics cause more sedation and orthostatic hypotension but less extrapyramidal symptoms than high-potency agents.
- Four main extrapyramidal symptoms: acute dystonia, akathisia, parkinsonism, and tardive dyskinesia (potentially irreversible).
- Neuroleptic malignant syndrome: fever, rigidity, altered mental status, autonomic instability; elevated CK; life-threatening emergency.
- Black box warning: increased mortality in elderly patients with dementia-related psychosis.
- Chlorpromazine causes photosensitivity; patients require sunscreen and protective clothing.
- Can cause QTc prolongation and torsades de pointes, especially with other QT-prolonging drugs or IV administration.
Indications
Chlorpromazine is FDA-approved for multiple psychiatric and non-psychiatric conditions:
Psychiatric indications
- Schizophrenia: management of acute psychotic episodes and maintenance therapy, though second-generation antipsychotics are now preferred first-line
- Acute mania: as part of bipolar disorder treatment, often combined with mood stabilizers
- Severe behavioral problems in children with conduct disorder or attention-deficit hyperactivity disorder (when other treatments have failed)
Non-psychiatric indications
- Intractable hiccups: often effective when other measures fail
- Severe nausea and vomiting: particularly in the perioperative setting or due to chemotherapy
- Acute intermittent porphyria: for symptomatic control during acute attacks
- Tetanus: as an adjunct to reduce muscle spasm and anxiety
Chlorpromazine is rarely used as first-line therapy today due to its unfavorable side effect profile compared to second-generation antipsychotics. However, it remains important for exam purposes and is still used in resource-limited settings due to low cost.
Pharmacokinetics
Chlorpromazine exhibits complex pharmacokinetics with significant interindividual variability:
Absorption: Well absorbed orally, but undergoes extensive first-pass metabolism, resulting in bioavailability of approximately 30-60%. Intramuscular administration bypasses first-pass effect and achieves higher plasma levels.
Distribution: Highly lipophilic with large volume of distribution. Extensively protein-bound (95-98%). Crosses the blood-brain barrier and placenta.
Metabolism: Extensively metabolized in the liver via multiple CYP450 enzymes (primarily CYP2D6, CYP1A2, CYP3A4). Forms numerous metabolites, some of which are pharmacologically active. This contributes to variable drug levels and unpredictable responses.
Excretion: Primarily renal excretion of metabolites. Half-life ranges from 16 to 30 hours, but active metabolites may persist much longer, leading to accumulation with chronic dosing.
Clinical implications: The long half-life allows once-daily or twice-daily dosing. Steady state is achieved after approximately 5-7 days. Hepatic impairment significantly affects clearance.
Adverse effects
Chlorpromazine has a broad adverse effect profile due to its multi-receptor antagonism:
Extrapyramidal symptoms (EPS)
- Acute dystonia: sustained muscle spasms, torticollis, oculogyric crisis (hours to days after starting)
- Akathisia: subjective restlessness and inability to sit still (days to weeks)
- Parkinsonism: bradykinesia, rigidity, tremor (weeks to months)
- Tardive dyskinesia: irreversible involuntary movements, particularly of the face and tongue (months to years; risk increases with duration of therapy)
Anticholinergic effects
- Dry mouth, blurred vision, constipation, urinary retention, confusion, and memory impairment (especially problematic in elderly patients)
Cardiovascular effects
- Orthostatic hypotension (alpha-1 blockade): most common cardiovascular effect, can cause falls
- QTc prolongation: dose-related risk of torsades de pointes, especially with IV administration or in combination with other QT-prolonging drugs
- Reflex tachycardia
Metabolic and endocrine effects
- Hyperprolactinemia (dopamine D2 blockade in tuberoinfundibular pathway): galactorrhea, amenorrhea, gynecomastia, sexual dysfunction
- Weight gain, hyperglycemia, dyslipidemia (less prominent than with many second-generation antipsychotics)
Neuroleptic malignant syndrome (NMS)
Rare but life-threatening emergency characterized by hyperthermia, muscle rigidity, altered mental status, and autonomic instability. Requires immediate discontinuation and supportive care.
Other notable effects
- Photosensitivity: patients develop severe sunburn easily; skin protection is essential
- Blue-gray skin discoloration: with chronic use
- Corneal and lens deposits: generally benign but require monitoring
- Cholestatic jaundice: rare, typically occurs in first few weeks
- Agranulocytosis: rare but serious; monitor for fever and sore throat
- Seizure threshold lowering: dose-related risk
- Sedation and drowsiness (H1 blockade)
Contraindications and warnings
Absolute contraindications
- Comatose states or severe CNS depression
- Concomitant use of large doses of other CNS depressants (alcohol, barbiturates, opioids)
- Known hypersensitivity to phenothiazines
- Bone marrow suppression
Relative contraindications and warnings
- Cardiovascular disease: use with caution due to orthostatic hypotension and QTc prolongation risk; obtain baseline ECG in at-risk patients
- Seizure disorders: lowers seizure threshold; may require anticonvulsant dose adjustment
- Parkinson disease: D2 blockade worsens motor symptoms
- Dementia-related psychosis: FDA black box warning for increased mortality in elderly patients with dementia (applies to all antipsychotics)
- Prostatic hypertrophy or urinary retention: anticholinergic effects worsen symptoms
- Narrow-angle glaucoma: anticholinergic effects increase intraocular pressure
- Hepatic impairment: reduced metabolism requires dose adjustment
- Pregnancy: crosses placenta; use only if benefit outweighs risk; third-trimester exposure may cause EPS in neonates
- Breastfeeding: excreted in breast milk; generally not recommended
Special populations
Elderly patients are at increased risk for falls (orthostatic hypotension), anticholinergic effects, EPS, and cerebrovascular events. Start with lower doses and titrate carefully.
Children may be more susceptible to dystonic reactions and should be monitored closely, especially during the first days of treatment.
Drug interactions
Chlorpromazine has numerous clinically significant drug interactions:
CNS depressants
Alcohol, benzodiazepines, opioids, barbiturates: additive CNS depression, increased sedation, and respiratory depression risk. Avoid concurrent use or use minimal doses with close monitoring.
Anticholinergic drugs
Antihistamines, tricyclic antidepressants, antiparkinson agents: additive anticholinergic effects including confusion, ileus, urinary retention, and hyperthermia. Particularly dangerous in elderly.
QTc-prolonging drugs
Class IA and III antiarrhythmics, macrolide antibiotics, fluoroquinolones, ondansetron, methadone: increased risk of torsades de pointes. Avoid combination or monitor ECG closely.
Antihypertensive agents
Alpha-blockers, beta-blockers, ACE inhibitors: additive hypotensive effects. Monitor blood pressure and adjust doses accordingly.
CYP450 interactions
- CYP2D6 inhibitors (fluoxetine, paroxetine, bupropion): increase chlorpromazine levels, raising toxicity risk
- CYP inducers (carbamazepine, phenytoin, rifampin): decrease chlorpromazine levels, reducing efficacy
Lithium
Rare reports of encephalopathy-like syndrome with combined use; monitor for confusion, tremor, and EPS.
Levodopa
Chlorpromazine antagonizes dopaminergic effects of levodopa, reducing its efficacy in Parkinson disease.
Epinephrine
Chlorpromazine may reverse epinephrine's vasopressor effects due to alpha-blockade, leading to paradoxical hypotension. Use norepinephrine instead if vasopressor is needed.
Clinical pearls
- Low-potency vs. high-potency antipsychotics: Chlorpromazine (low-potency) causes more sedation, anticholinergic effects, and orthostatic hypotension but less EPS compared to high-potency agents like haloperidol. Remember the trade-off: low-potency = more autonomic side effects, less EPS; high-potency = more EPS, fewer autonomic effects.
- Acute dystonia management: Treat with benztropine (anticholinergic) or diphenhydramine (antihistaminic with anticholinergic properties) intramuscularly or intravenously for rapid relief.
- NMS recognition: Classic tetrad is fever, rigidity, altered mental status, and autonomic instability. Laboratory findings include elevated creatine kinase (often >1000 U/L), leukocytosis, and myoglobinuria. Stop chlorpromazine immediately and provide supportive care; consider dantrolene or bromocriptine in severe cases.
- Tardive dyskinesia: Risk increases with cumulative exposure. The condition may be irreversible even after discontinuation. AIMS (Abnormal Involuntary Movement Scale) screening should be performed at baseline and periodically. Valbenazine and deutetrabenazine are FDA-approved treatments.
- Chlorpromazine equivalents: Used to compare antipsychotic doses. 100 mg chlorpromazine is equivalent to approximately 2 mg haloperidol or 2 mg risperidone.
- Sunscreen counseling: Patients must use high-SPF sunscreen and protective clothing due to significant photosensitivity. This is a board-favorite counseling point.
- IM administration caution: Intramuscular injection can cause severe orthostatic hypotension. Keep patient supine for at least 30 minutes after injection and monitor blood pressure.
- Not first-line anymore: While historically important as the first antipsychotic (1950s), chlorpromazine is now rarely first-line due to side effect burden. Second-generation antipsychotics are generally preferred, but chlorpromazine remains in use for cost reasons and specific indications like intractable hiccups.
- Metabolic monitoring: Although less metabolically problematic than some second-generation antipsychotics (olanzapine, clozapine), patients should still have baseline and periodic monitoring of weight, fasting glucose, and lipids.
- Blue-gray pigmentation: Long-term use (years) may cause blue-gray discoloration of sun-exposed skin. This is a classic exam finding and is generally irreversible.
Low-potency antipsychotics (Chlorpromazine, Thioridazine) = "Cheating Thief" = more Cholinergic blockade (anticholinergic effects), more Histamine blockade (sedation), more hTension (orthostatic hypotension). High-potency = more EPS.
Practice questions
1. A 28-year-old man with schizophrenia is started on chlorpromazine for acute psychosis. Three days later, he presents to the emergency department with sustained contraction of his neck muscles, causing his head to twist to the right. His temperature is 37.2°C (99°F), blood pressure is 128/82 mm Hg, and pulse is 88/min. Physical examination shows marked cervical muscle rigidity and torticollis. Which of the following is the most appropriate immediate treatment?
2. A 67-year-old woman with a 20-year history of schizophrenia treated with chlorpromazine presents for routine follow-up. She has involuntary lip-smacking movements and tongue protrusion that worsen when she is not concentrating on suppressing them. Her medication regimen has remained unchanged for the past 10 years. Vital signs are normal. Mental status examination shows no acute psychotic symptoms. Which of the following is the most likely diagnosis?
3. A 34-year-old man is brought to the emergency department by his family with confusion and fever. He was started on chlorpromazine 5 days ago for acute psychosis. His temperature is 40°C (104°F), blood pressure is 168/96 mm Hg, pulse is 122/min, and respirations are 24/min. Physical examination shows profuse diaphoresis, generalized muscle rigidity, and fluctuating level of consciousness. Laboratory studies show a creatine kinase of 8,500 U/L and white blood cell count of 15,000/mm³. Which of the following is the most appropriate immediate management?
Frequently asked questions
How does chlorpromazine work?
Chlorpromazine works primarily by blocking dopamine D2 receptors in the brain's mesolimbic pathway, reducing positive psychotic symptoms like hallucinations and delusions. It also blocks histamine H1, muscarinic acetylcholine, and alpha-1 adrenergic receptors, which accounts for its sedating, anticholinergic, and hypotensive side effects. As a low-potency antipsychotic, it requires higher doses than high-potency agents and has a broader side effect profile.
What are the side effects of chlorpromazine?
Common side effects include extrapyramidal symptoms (dystonia, akathisia, parkinsonism, tardive dyskinesia), sedation, dry mouth, blurred vision, constipation, urinary retention, and orthostatic hypotension. It can cause significant weight gain, hyperprolactinemia (leading to galactorrhea and sexual dysfunction), and photosensitivity requiring sun protection. Serious but rare adverse effects include neuroleptic malignant syndrome, QTc prolongation with risk of arrhythmias, agranulocytosis, and cholestatic jaundice.
Is chlorpromazine a controlled substance?
No, chlorpromazine is not a controlled substance. It is a prescription-only antipsychotic medication that does not have significant abuse potential and is not scheduled under the Controlled Substances Act. However, it requires careful monitoring due to its serious side effect profile and potential for adverse reactions.
What is the difference between chlorpromazine and haloperidol?
Chlorpromazine is a low-potency first-generation antipsychotic while haloperidol is high-potency. Chlorpromazine causes more sedation, anticholinergic effects (dry mouth, constipation, urinary retention), and orthostatic hypotension but produces fewer extrapyramidal symptoms at therapeutic doses. Haloperidol has greater D2 receptor selectivity, causes more extrapyramidal symptoms but fewer autonomic side effects, and requires much lower doses (2 mg haloperidol ≈ 100 mg chlorpromazine). Both can cause tardive dyskinesia and neuroleptic malignant syndrome with chronic use.
Why is chlorpromazine used for intractable hiccups?
Chlorpromazine is FDA-approved for intractable hiccups and is often effective when other measures fail. Its mechanism for hiccup suppression is not fully understood but likely involves dopamine receptor blockade in the medulla, relaxation of the diaphragm through central nervous system depression, and possible effects on the phrenic nerve reflex arc. It is typically reserved for severe, persistent hiccups that have not responded to simpler interventions like breath-holding maneuvers or other medications.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: N05AA01
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.