The "QT interval" is a measurement on an electrocardiogram (ECG) that reflects how long the heart takes to electrically reset between beats. When it gets too long — "QT prolongation" — it can predispose to a dangerous arrhythmia called torsades de pointes, which can cause sudden death. Several antipsychotics modestly prolong the QT interval. The clinical risk is small for most people, but for some it is significant, and understanding when monitoring matters is part of safe prescribing.
QT prolongation is a measurable but usually small effect of certain antipsychotics — significant risk concentrates in people with other risk factors (electrolyte disturbance, other QT-prolonging drugs, structural heart disease).
What QT prolongation actually is
The QT interval is measured from the start of ventricular depolarisation (the Q wave) to the end of repolarisation (the T wave). Because heart rate affects it, clinicians use the rate-corrected version (QTc). Roughly:
- Normal QTc: under 440 ms in men, under 460 ms in women
- Borderline: 440–470 ms (men), 460–480 ms (women)
- Prolonged: above 470 ms (men), above 480 ms (women)
- Concerning: above 500 ms — risk of torsades rises notably
Which antipsychotics prolong QT most
Based on FDA labelling and pooled studies:
- Highest: thioridazine, mesoridazine, droperidol, pimozide, sertindole — most carry specific FDA warnings, and several are now rarely used
- Moderate (clinically relevant): ziprasidone, iloperidone, IV haloperidol, paliperidone
- Lower: oral haloperidol, risperidone, olanzapine, quetiapine, clozapine
- Lowest: aripiprazole, lurasidone, brexpiprazole, cariprazine
Ziprasidone in particular has a well-known QT signal — the FDA label includes specific monitoring guidance and a list of contraindicated co-medications. The CATIE trial included regular ECG monitoring partly to characterise this.
Who is at increased risk
- People with congenital long QT syndrome
- People with low potassium or low magnesium (e.g. from diuretics, eating disorders, vomiting, diarrhoea)
- Older adults
- People with structural heart disease, heart failure, or a history of arrhythmia
- People taking other QT-prolonging drugs (certain antibiotics like azithromycin, antifungals, some antidepressants like citalopram, methadone)
- People with bradycardia (slow heart rate)
The combination of factors matters more than any single one. Most arrhythmias from QT prolongation occur when several risk factors stack.
How risky is it really?
The absolute risk of torsades de pointes from a typical antipsychotic in a typical patient is very low — estimated at well under 1 per 1,000 patient-years for most agents. Sudden cardiac death rates are slightly elevated in antipsychotic-treated populations compared to controls, but the absolute risk per individual is small. The risk is not zero, however, and it concentrates in identifiable higher-risk subgroups — which is why screening and monitoring matter.
Symptoms to watch for
QT prolongation itself is silent. Torsades de pointes, when it occurs, can cause:
- Palpitations
- Lightheadedness
- Fainting (syncope)
- Sudden cardiac arrest
Monitoring that makes sense
There is no single global standard, but reasonable practice includes:
- A baseline ECG before starting QT-prolonging drugs (especially ziprasidone, IV haloperidol) in patients with cardiac risk factors
- Repeat ECG after dose stabilisation, particularly with significant dose increases
- Baseline electrolytes (potassium, magnesium) and correction of any deficiency before starting
- Particular caution if combining with other QT-prolonging medications
- For low-risk patients on low-risk antipsychotics, routine ECG monitoring is not generally recommended
The European Medicines Agency and FDA have both issued specific cautions for thioridazine, droperidol, and (to a lesser extent) ziprasidone and IV haloperidol.
What to do if your QTc is high
Decisions are individualised and made with a prescriber, often in consultation with cardiology. Possible steps:
- Correct any electrolyte abnormalities (replace potassium and magnesium)
- Stop or substitute any other QT-prolonging medications
- Lower the antipsychotic dose
- Switch to a lower-QT-risk antipsychotic (aripiprazole, lurasidone, brexpiprazole, cariprazine)
- For severe prolongation (over 500 ms), discontinue the offending drug urgently
When to call your prescriber
Sudden palpitations, fainting, severe dizziness, or chest discomfort — particularly when starting or changing a QT-prolonging antipsychotic. Family history of sudden cardiac death or known long QT syndrome should be discussed before starting any antipsychotic. New medications added by other prescribers (antibiotics, antifungals, methadone) should always be cross-checked with your psychiatrist for QT interactions.
Putting it in perspective
For most patients on most antipsychotics, QT prolongation is a small, manageable issue. For a smaller group with stacked risk factors, it can be a real safety problem — and one that is preventable with the right baseline checks and the right drug choices. The right answer is not fear, and not blanket avoidance of QT-affecting drugs (some, like ziprasidone, are highly effective and well-tolerated in many other ways), but informed risk management.
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.