Quetiapine — Seroquel — has the unusual property of behaving like different drugs at different doses. The mechanism is dose-dependent in a way that creates three clinically distinct regimes from one molecule.
- Class
- Second-generation antipsychotic
- Mechanism
- D2 + 5-HT2A + H1 + alpha-1 + 5-HT1A partial agonism. Active metabolite norquetiapine has NET inhibition (contributing to bipolar depression effect).
- Typical dose
- Schizophrenia 400-800 mg/day; bipolar mania 400-800; bipolar depression 300; MDD augmentation 150-300. Often XR formulation.
- Half-life
- ~6 hours (IR); XR extends duration
- FDA indications
- Schizophrenia, bipolar I (mania, depression, maintenance), MDD adjunct
- Key adverse effects
- Sedation (prominent, especially at low doses), weight gain (moderate-substantial), metabolic effects, orthostasis, dry mouth, low EPS rate (lowest among SGAs)
Black box: Increased mortality in elderly patients with dementia-related psychosis
Widely used off-label for insomnia at low doses (25-100 mg) — controversial given metabolic burden of an antipsychotic for non-psychiatric sleep. Antipsychotic effects require ≥300 mg. The breadth of bipolar approvals is distinguishing.
At 25-100 milligrams, H1 antihistamine activity dominates. The patient experiences primarily sedation with minimal antipsychotic effect. This is the dose range used off-label for insomnia — controversial because it puts patients on an antipsychotic for a sleep problem.
At 150-300 milligrams, the 5-HT2A blockade and NET inhibition (via the active metabolite norquetiapine) become significant. This is the dose range with antidepressant effect, used for bipolar depression and as MDD adjunct.
At 400-800 milligrams, full D2 antagonism appears and quetiapine becomes a proper antipsychotic. This is the dose range for schizophrenia and acute mania.
One drug, three jobs. FDA approvals span the range: schizophrenia, bipolar I (mania, depression, maintenance), and MDD adjunct. The bipolar breadth — covering mania, depression, and maintenance from one agent — is rare.
Quetiapine is the clearest single-drug illustration of dose-dependent receptor recruitment — its clinical identity changes completely across its dose range.
Mechanism note: Quetiapine is the canonical sequential-binding drug — antihistamine low, antidepressant mid, antipsychotic high. The dose IS the diagnosis of what the drug is doing.
EPS rate is the lowest in the SGA class. Quetiapine's loose D2 binding produces minimal nigrostriatal blockade. For patients with prior EPS history or particular EPS sensitivity, quetiapine is often a safe choice.
The trade-offs are metabolic and sedation. Quetiapine causes meaningful weight gain (less than olanzapine, more than aripiprazole or lurasidone), lipid elevation, and glucose dysregulation. The sedation is profound at low doses and significant at higher ones. The off-label sleep use is widespread but carries antipsychotic-class warnings and metabolic concerns even at low doses; trazodone, melatonin agonists, or behavioral interventions are often preferable for primary insomnia.
- Cost
- Generic IR/XR: ~$15-50/month.
- Generic status
- Generic since 2012 (IR) and 2017 (XR).
- Formulary typical
- Tier 1-2 generic. No PA.
- Access friction
- None.
Prescriber tip: Cheap and accessible. The off-label sleep use at 25-100 mg is widespread but carries class-warning concerns; weigh against alternatives.
For bipolar disorder across the spectrum, quetiapine remains a leading choice when its metabolic profile is acceptable. For metabolically vulnerable patients, lurasidone, cariprazine, or lumateperone serve the same coverage with less burden.