Haloperidol vs Olanzapine vs Quetiapine

Current · V5 (2025) → H6.v 1 exam appearance

2026A Q10

Exam question

Compare and contrast the pharmacodynamics of haloperidol, olanzapine, and quetiapine, relevant to their use in intensive care, using the following headings:
a) primary mechanism of action (25% of marks).
b) extrapyramidal side effects (15% of marks).
c) other receptor mediated effects (30% of marks).
d) cardiac toxicity (15% of marks).
e) idiosyncratic adverse effects (15% of marks).

CICMWrecks answer

Master answer

Compare Haloperidol, Olanzapine & Quetiapine in the Pharmacopeia

a) Primary mechanism of action (25%)

Drug	Primary pharmacodynamic mechanism	ICU-relevant consequence
Haloperidol	First-generation antipsychotic with high-affinity dopamine D₂ receptor antagonism, particularly in mesolimbic pathways	Potent antipsychotic effect, but stronger D₂ blockade also drives extrapyramidal toxicity, hyperprolactinaemia and higher NMS risk
Olanzapine	Second-generation antipsychotic with antagonism at both 5-HT_2A and D₂ receptors; relatively greater serotonergic than D₂ effect than haloperidol	Antipsychotic/sedative effect with lower EPS risk than haloperidol but more metabolic, H₁, α₁ and muscarinic adverse effects
Quetiapine	Second-generation antipsychotic with 5-HT_2A antagonism greater than D₂ antagonism; active norquetiapine also contributes to receptor effects	Lowest D₂-mediated EPS burden of the three; prominent sedation and orthostatic hypotension

b) Extrapyramidal side effects (15%)

EPS arise predominantly from D₂ blockade in the nigrostriatal pathway.

Feature	Haloperidol	Olanzapine	Quetiapine
Relative risk	Highest	Low–moderate	Lowest
Why	High D₂ affinity / sustained blockade	Lower D₂ affinity with stronger 5-HT_2A antagonism	Low D₂ affinity and relatively greater 5-HT_2A antagonism
Examples	Acute dystonia (e.g. torticollis, oculogyric crisis), akathisia, drug-induced parkinsonism, and tardive dyskinesia

c) Other receptor-mediated effects (30%)

Receptor / pathway	Haloperidol	Olanzapine	Quetiapine
H₁ antagonism	Relatively weak	Prominent → sedation, ↑ appetite / weight gain	Prominent → sedation, somnolence, ↑ appetite
α₁ antagonism	Some activity → hypotension, usually less prominent	Orthostatic hypotension	Prominent orthostatic hypotension / dizziness
Muscarinic antagonism	Minimal	Present → dry mouth, constipation, urinary retention, blurred vision, confusion	Parent drug has little M₁ affinity, but norquetiapine has clinically relevant antimuscarinic activity → similar effects
D₂ in tuberoinfundibular pathway	Hyperprolactinaemia relatively common	Less prominent	Generally low risk
5-HT_2C / H₁ related metabolic effects	Less prominent	Greatest metabolic burden → weight gain, hyperglycaemia, dyslipidaemia	Moderate metabolic burden
Clinical sedation	Usually less sedating	Moderate–marked	Marked, often useful where sedating effect is desired

d) Cardiac toxicity (15%)

All three can prolong the QT interval, principally through blockade of the cardiac delayed rectifier K⁺ current (hERG / I_Kr), predisposing to torsades de pointes.
Haloperidol has the greatest clinically important QT risk of the three; risk rises with higher doses, parenteral use, electrolyte disturbance, structural heart disease and other QT-prolonging drugs.
Quetiapine: QT prolongation is usually modest at therapeutic doses but becomes more relevant with overdose, interacting drugs, electrolyte disturbance or other risk factors.
Olanzapine: generally lower QT liability than haloperidol, but QT prolongation can still occur.
α₁ blockade with olanzapine and especially quetiapine also contributes to orthostatic hypotension.

e) Idiosyncratic adverse effects (15%)

Adverse effect	Comparison
Neuroleptic malignant syndrome	Can occur with all three; risk is greatest with haloperidol. Features: hyperthermia, rigidity, autonomic instability, altered consciousness, ↑ CK / rhabdomyolysis
Blood dyscrasias	Leukopenia, neutropenia and rarely agranulocytosis can occur with all antipsychotics
Seizures	All may lower seizure threshold; clinically more relevant with atypical agents and predisposed patients
Severe hypersensitivity / DRESS	Rare but reported, particularly with atypical antipsychotics

ICU comparison — practical implications

Haloperidol: strongest D₂ antagonist; lowest metabolic burden but highest EPS, NMS and QT concern.
Olanzapine: lower EPS burden, but more sedation, anticholinergic effects and the greatest metabolic adverse-effect burden.
Quetiapine: lowest EPS burden and often most sedating; hypotension and somnolence are prominent, with some antimuscarinic effect via norquetiapine.
For ICU use, drug choice should consider QT interval, haemodynamics, desired sedation, existing Parkinsonism/EPS risk, anticholinergic burden and metabolic vulnerability.

Quick reference

Summary

Feature	Haloperidol	Olanzapine	Quetiapine
Core mechanism	High-affinity D2 antagonist	5-HT2A + D2 antagonist	5-HT2A > D2 antagonist
EPS	Highest	Low–moderate	Lowest
Sedation	Less	Moderate–marked	Marked
Antimuscarinic	Minimal	Present	Via norquetiapine
Metabolic burden	Low	Highest	Moderate
QT risk	Highest	Lower	Lower–moderate
NMS	Highest risk	Possible	Possible

Past papers

Exam appearances

1 appearance

Exam	Exact exam wording	Candidate success
2026A Q10	Compare and contrast the pharmacodynamics of haloperidol, olanzapine, and quetiapine, relevant to their use in intensive care, using the following headings: a) primary mechanism of action (25% of marks). b) extrapyramidal side effects (15% of marks). c) other receptor mediated effects (30% of marks). d) cardiac toxicity (15% of marks). e) idiosyncratic adverse effects (15% of marks).	21.7%