QT prolongation and antipsychotics

The QT interval is the time on an electrocardiogram (ECG) between the start of the heart's electrical depolarization and the end of repolarization — essentially, how long it takes the ventricles to reset for the next beat. When the QT interval gets too long, the heart becomes vulnerable to a dangerous arrhythmia called torsades de pointes, which can degenerate into ventricular fibrillation and sudden cardiac death. Several antipsychotics lengthen the QT interval, and at least two have been pulled from major markets specifically because of cardiac risk.

The mechanism

Most QT-prolonging drugs block the rapid component of the delayed rectifier potassium current (IKr), encoded by the hERG gene. Blocking IKr slows ventricular repolarization, which lengthens the QT interval on ECG. The same channel is blocked by many non-cardiac drugs (some antibiotics, some antifungals, some antiemetics), which is why drug-drug combinations are often the trigger for clinically significant prolongation.

What QTc means

The QT interval shortens at higher heart rates, so it is corrected for rate to give the QTc. The most common formula is Bazett's. General reference ranges:

• Normal: QTc <450 ms (men), <460 ms (women)
• Borderline: 450–470 ms (men), 460–480 ms (women)
• Prolonged: >470 ms (men), >480 ms (women)
• Concerning: >500 ms — risk of torsades rises markedly
• An increase of >60 ms from baseline is also clinically significant

Antipsychotics ranked by QT effect

• Highest: thioridazine (largely withdrawn for this reason), pimozide, IV haloperidol, ziprasidone
• Moderate: iloperidone, quetiapine (especially overdose), chlorpromazine
• Low to moderate: risperidone, paliperidone, olanzapine, clozapine, asenapine
• Low: aripiprazole, brexpiprazole, cariprazine, lurasidone, lumateperone

Numbers come from regulatory data and meta-analyses. The size of the average QTc increase on standard antipsychotic doses is typically modest (5–20 ms), but individual responses vary, and combinations push the total higher.

The thioridazine and droperidol stories

Thioridazine (Mellaril) was a widely used first-generation antipsychotic until reports of QT prolongation, torsades, and sudden death led to a 2000 FDA boxed warning and effective withdrawal from clinical use. Droperidol, a butyrophenone used in emergency departments and anaesthesia, received a similar boxed warning in 2001, which sharply reduced its use. Both episodes shaped the modern attention to QT in antipsychotic prescribing.

Risk factors that compound the danger

• Bradycardia (slow heart rate)
• Hypokalemia (low potassium)
• Hypomagnesemia (low magnesium)
• Hypocalcemia
• Female sex
• Older age
• Pre-existing heart disease, especially heart failure
• Congenital long QT syndrome
• Concurrent QT-prolonging medications (some antibiotics like azithromycin and erythromycin, antifungals like fluconazole, antiemetics like ondansetron, methadone, several antiarrhythmics)
• Hepatic or renal impairment that raises drug levels

Monitoring

The general approach, summarised in FDA labels, the NICE schizophrenia guideline, and reviews such as Maudsley Prescribing Guidelines:

• Baseline ECG before starting high-risk agents (ziprasidone, IV haloperidol, pimozide, iloperidone)
• Repeat ECG after dose stabilization on high-risk agents
• ECG before starting any antipsychotic in patients with cardiac history, electrolyte abnormalities, or other QT-prolonging medications
• Check potassium and magnesium periodically
• Avoid combinations of multiple QT-prolonging drugs when possible

Routine ECG monitoring on aripiprazole or lurasidone in a healthy young adult is generally not required.

What to do if QTc is prolonged

1. Repeat the ECG to confirm
2. Check and correct potassium and magnesium
3. Review all medications for additive QT effects
4. Consider lowering the antipsychotic dose
5. If QTc remains >500 ms or has risen by >60 ms from baseline, switch to a lower-risk agent
6. Cardiology consultation when the picture is complex

The clozapine special case

Clozapine has only modest direct QT effects but is associated with myocarditis and cardiomyopathy, which can present with chest pain, shortness of breath, and ECG abnormalities. Baseline and follow-up ECG plus troponin and inflammatory markers are part of clozapine initiation in many protocols. See our clozapine side effects guide.

Practical principles

• If you have heart disease, family history of sudden cardiac death, or are on other QT-prolonging medications, mention this before starting any antipsychotic.
• Ask whether a baseline ECG is part of your starting protocol.
• If you ever feel sudden palpitations, dizziness, or fainting on an antipsychotic, seek evaluation rather than waiting for the next routine visit.
• Be aware that some over-the-counter and prescribed medications can interact additively. Tell every prescriber the full medication list.

The bottom line

QT prolongation from antipsychotics is, for most patients on most agents, a small risk that monitoring and good prescribing practice keep manageable. The risk is concentrated in particular medications and particular patients — older adults with heart disease, patients on multiple QT-active drugs, patients with low potassium or magnesium. Knowing where the risk lives makes it easier to address. The era of thioridazine and IV droperidol shaped the modern caution, and that caution has saved lives.

For more, see our QT prolongation overview, ECG monitoring guide, and orthostatic hypotension.

This article is for educational purposes only and is not medical advice, diagnosis, or treatment. Always consult a qualified mental health professional. If you or someone you know is in crisis, call or text 988 in the US, or your local emergency number.