SGA / Dibenzothiazepine·Psychiatry - Antipsychotics

QUETIAPine

Schizophrenia, Bipolar mania, Bipolar depression

Authority requiredPBSSGASeroquel, Seroquel XR300 mg tablet+2

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At a glance

Max Dose

800 mg/day

Half-life

7 h

Caution: Sedation

SEVERE

Caution: Orthostasis

HIGH RISK

Cautions

Avoid / Cautions

Concurrent use of strong CYP3A4 inhibitors (Clarithromycin, Ketoconazole).

Where it stands

Rapid Summary

Overview

SGA with extremely strong histamine and alpha-1 blockade. Functions as a potent sedative at low doses, an antidepressant at moderate doses, and an antipsychotic at high doses.

Clinical Focus

Severe risk of orthostatic hypotension and falls, particularly when initiating or escalating the dose.

Bottom Line

Quetiapine is a widely prescribed atypical antipsychotic used across psychosis, bipolar, and off-label insomnia, but causes significant metabolic effects and sedation, and off-label low-dose use should be critically reviewed.

Formulation & Access

Brand Names

Seroquel, Seroquel XR

Oral Routes

Do not crush

IR Tablets (25, 100, 200, 300 mg). XR Tablets (50, 150, 200, 300, 400 mg). Do NOT crush XR.

Prescribing & PBS

Authority Required (PBS).

Indications & Use

Primary

PBSTGA

Schizophrenia, Bipolar mania, Bipolar depression.

Off-Label

OFF

Severe insomnia, borderline personality disorder, generalized anxiety.

Clinical Place

The most widely prescribed 'off-label' sedative in hospitals. Essential for Bipolar depression.

Quick reference

Class & Role

SGA / Dibenzothiazepine — SGA with extremely strong histamine and alpha-1 blockade.

Route / Formulation

IR Tablets (25, 100, 200, 300 mg). XR Tablets (50, 150, 200, 300, 400 mg). Do NOT crush XR. (Seroquel, Seroquel XR)

Usual Dose & Max

PO 12.5 - 25 mg NOCTE. Max 800 mg/day.

Key Indication Doses

Insomnia / Anxiety (Off-Label): PO 12.5 - 25 mg NOCTE. Bipolar Depression: PO 300 mg NOCTE (Often using XR formulations). Schizophrenia / Mania: PO 400 - 800 mg/day in divided doses or as a single XR dose at night. Elderly: Start at 12.5 mg NOCTE. Max 100 mg/day in elderly. Highly sensitive to alpha-1 mediated falls.

Best Uses

Key Risks

Cardiovascular: Orthostatic hypotension, tachycardia, QTc prolongation. Neurological: Somnolence, dizziness, 'hangover' effect. Metabolic: Weight gain, hyperglycemia, hyperlipidemia (second only to Olanzapine/Clozapine).

Key Interactions

CYP3A4 inhibitors massively spike Quetiapine levels, causing profound sedation, coma, and QTc prolongation. CYP3A4 inducers (Phenytoin) render it useless. Additive hypotension with antihypertensives.

Monitoring

Postural BP checks for the first 3 days of therapy or upon dose increase. Baseline BSL, HbA1c, Lipids, and Weight.

Clinical Pearl

Quetiapine is 3 different drugs depending on the dose. At 25mg, it only blocks histamine (it's a sleeping pill). At 300mg, it blocks NET and 5HT2A (it's an antidepressant). You must hit 400-800mg to reliably block D2 receptors (it becomes an antipsychotic).

Access

Authority requiredSeroquel, Seroquel XR300 mg tablet

Brand: Seroquel, Seroquel XR
PBS status: Authority required
Formulation: IR Tablets (25, 100, 200, 300 mg). XR Tablets (50, 150, 200, 300, 400 mg). Do NOT crush XR.

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