Quetiapine – Uses, Mechanism & Side Effects

What is Quetiapine?

Quetiapine is an atypical antipsychotic belonging to the dibenzothiazepine class of medications. It is used to manage several serious mental health conditions and is marketed under the brand name Seroquel, as well as available in generic form. Quetiapine was developed in the 1990s and is now one of the most widely prescribed antipsychotic medications worldwide.

Mechanism of Action

Quetiapine exerts its therapeutic effects by acting as an antagonist (blocker) at multiple receptors in the central nervous system (CNS):

• Dopamine D2 receptors: Blocking these receptors reduces excessive dopamine activity, which plays a key role in schizophrenia and mania.
• Serotonin 5-HT2A receptors: Inhibition of these receptors contributes to mood-stabilizing and antidepressant effects and reduces the risk of movement side effects.
• Histamine H1 receptors: Blockade at these receptors accounts for the sedating and sleep-promoting properties of the drug.
• Alpha-1 adrenergic receptors: Inhibition can lead to blood pressure-lowering effects.

The active metabolite norquetiapine also inhibits norepinephrine reuptake, contributing further to the antidepressant efficacy of quetiapine.

Indications

Quetiapine is approved for the following conditions:

• Schizophrenia: Treatment of both positive symptoms (e.g., hallucinations, delusions) and negative symptoms (e.g., social withdrawal, blunted affect)
• Bipolar disorder: Treatment of manic episodes, depressive episodes associated with bipolar disorder, and prevention of relapse (mood stabilization)
• Major depressive disorder (MDD): As an add-on (augmentation) therapy when antidepressants alone are insufficient (extended-release formulation)

Dosage and Administration

Quetiapine is available as an immediate-release tablet (quetiapine IR, taken multiple times daily) and as an extended-release tablet (quetiapine XR, taken once daily). Dosing depends on the condition being treated:

• Schizophrenia: Typical daily dose 300–800 mg
• Bipolar mania: Typical daily dose 400–800 mg
• Bipolar depression: Typical daily dose 300 mg
• Augmentation in MDD: Lower doses of 50–300 mg per day

The dose is always initiated low and gradually increased under medical supervision. Discontinuation should also be done gradually to avoid withdrawal symptoms.

Side Effects

Like all medications, quetiapine can cause side effects. Common side effects include:

• Sedation and drowsiness (especially at the start of treatment)
• Weight gain and metabolic changes (elevated blood sugar and lipid levels)
• Dizziness and orthostatic hypotension (a drop in blood pressure upon standing)
• Dry mouth
• Constipation

Less common but clinically important side effects include:

• Elevated blood glucose levels or new-onset diabetes mellitus
• QTc interval prolongation on ECG (risk of cardiac arrhythmias)
• Tardive dyskinesia (involuntary movements, less common than with typical antipsychotics)
• Cataract formation – regular eye examinations are recommended during long-term use

Drug Interactions

Quetiapine can interact with a number of other medications. Particular caution is advised with:

• Drugs that inhibit or induce the CYP3A4 enzyme (e.g., ketoconazole, carbamazepine, rifampicin)
• Other central nervous system depressants (alcohol, benzodiazepines, opioids)
• Medications that prolong the QTc interval

Contraindications

Quetiapine should not be used in patients with:

• Known hypersensitivity to quetiapine or any of the excipients
• Concomitant use of potent CYP3A4 inhibitors such as HIV protease inhibitors or azole antifungals

In elderly patients with dementia-related psychosis, quetiapine should be used with great caution due to an increased risk of stroke and mortality.

Patient Information

• Quetiapine is a prescription-only medication and must only be taken as directed by a doctor.
• It should be taken regularly and at the same time each day.
• Alcohol should be avoided during treatment.
• Use during pregnancy and breastfeeding requires careful risk-benefit assessment by a healthcare professional.
• Regular monitoring (blood glucose, lipids, body weight, ECG, eye examination) is recommended during long-term therapy.

References

1. Summary of Product Characteristics – Seroquel (Quetiapine), AstraZeneca, current approved version (European Medicines Agency, EMA).
2. National Institute for Health and Care Excellence (NICE) – Psychosis and Schizophrenia in Adults: Prevention and Management. Clinical Guideline CG178, 2014.
3. Stahl, S. M. – Stahl's Essential Psychopharmacology: Neuroscientific Basis and Practical Applications, 4th Edition, Cambridge University Press, 2013.