Thioridazine (Mellaril)

What it treats

Thioridazine is FDA-approved for schizophrenia in adults who have failed to respond adequately to other antipsychotics. That "second-line only" framing was added to the label after the QTc risk became clear. It's not a starting drug for a new episode of psychosis, and it's not a maintenance drug for most people. It's reserved for specific situations where the risk-benefit calculation still favors trying it.

How it works

Thioridazine is a low-potency phenothiazine that blocks dopamine at the D2 receptor. It has strong additional activity at muscarinic, alpha-adrenergic, and histamine receptors, which is why it's sedating and orthostatic. Its QTc effect appears to reflect blockade of cardiac potassium channels, and it's more prominent than with most other antipsychotics.

Receptor mechanism (detail)

Thioridazine is a D2 antagonist with strong anticholinergic and alpha-1 blocking activity and a distinctive effect on cardiac hERG potassium channels. That hERG activity is what drives the QTc prolongation seen on ECG. It also blocks histamine H1 strongly, contributing to sedation and appetite increase.

Potency and typical dosing pattern

Ranges are typical framework only, not a prescription for any individual. Thioridazine is low-potency, so milligram numbers are high.

For refractory schizophrenia, doses typically range from 50 to 800 mg per day, divided in two to four doses. Higher doses raise QTc risk further and are also linked to pigmentary retinopathy. The prescriber sets the dose based on response, side effects, and ECG.

Safety monitoring

• ECG at baseline and periodically throughout treatment. This is central to using thioridazine safely.
• Potassium and magnesium at baseline, low levels raise QTc risk.
• Ophthalmologic exam at baseline and periodically, especially if doses exceed 800 mg per day, to screen for pigmentary retinopathy.
• Involuntary-movement screen (AIMS) every six months.
• Metabolic labs, weight, BMI, fasting glucose or HbA1c, and lipids at baseline and annually.
• Blood pressure and orthostatic vitals, especially in older adults.
• Liver function at baseline and if symptoms suggest a problem.

Metformin co-commencement: Aoife Carolan / Schizophrenia Bulletin guideline.

A clinical guideline led by Aoife Carolan strongly recommends co-commencing metformin alongside high-risk antipsychotics like olanzapine or clozapine. This proactive approach helps mitigate severe metabolic side effects, significantly reducing antipsychotic-induced weight gain and improving insulin resistance. The Schizophrenia Bulletin guideline states that when prescribing olanzapine or clozapine, metformin should be initiated immediately to prevent weight gain and cardiometabolic issues. Thioridazine is a first-generation antipsychotic and isn't on the automatic co-commencement list, but metformin is recommended if weight rises more than 3 percent of pre-medication weight or if other cardiometabolic risk factors are present.

Typical titration used in the guideline: 500 mg once daily, then 500 mg twice daily after one week, then 500 mg increments every two weeks as tolerated, up to 1000 mg twice daily by about week six. Contraindicated with eGFR below 30 mL/min/1.73 m². Renal function is checked annually and metformin is held during acute illness or dehydration.

Source: Carolan A, et al. Metformin for the Prevention of Antipsychotic-Induced Weight Gain: Guideline Development and Consensus Validation. Schizophrenia Bulletin. 2025;51(5):1193 to 1203.

What to expect

Common side effects

• Sedation, often prominent.
• Orthostatic hypotension.
• Dry mouth, constipation, blurred vision, urinary hesitancy.
• Weight gain.
• Retrograde ejaculation.
• Photosensitivity.

Serious side effects and warnings

Boxed warning. Thioridazine carries prominent boxed warnings for QTc prolongation and torsades de pointes, and for the general antipsychotic warning about increased mortality in older adults with dementia-related psychosis. The label specifies that thioridazine is reserved for people who haven't responded adequately to other antipsychotics.

• QTc prolongation and arrhythmia risk. The central safety concern. Baseline and periodic ECGs are essential. Combining thioridazine with other QTc-prolonging drugs, CYP2D6 inhibitors, or drugs that lower potassium can be dangerous.
• Pigmentary retinopathy. A distinctive risk of thioridazine, more common at doses above 800 mg per day. Ophthalmologic screening is part of the safety plan.
• Tardive dyskinesia. Risk rises with dose and duration.
• Neuroleptic malignant syndrome. Rare but serious.
• Agranulocytosis. Rare but described.

This isn't medical advice. Any concern about a serious side effect should be raised with a prescriber promptly.

Sexual side effects

Thioridazine is notorious for sexual side effects, including retrograde ejaculation, delayed orgasm, and lowered libido. Some of this comes from strong alpha-1 blockade. If sexual side effects appear, it's worth raising with the prescriber.

Weight, appetite, and sleep

Weight gain is common. Sedation often helps sleep, and many people take the largest dose at bedtime. Daytime sedation can be a limiting factor.

Starting and dosing basics

This section is general background, not a dosing instruction. Thioridazine comes as tablets and an oral concentrate. It can be taken with or without food. Sun protection matters because of photosensitivity.

Missed doses and interactions

If you miss a dose, take it when you remember unless it's almost time for the next one, then skip and carry on. Don't double up.

Drug interactions matter more with thioridazine than with most antipsychotics. CYP2D6 inhibitors (fluoxetine, paroxetine, bupropion) can raise thioridazine levels. QTc-prolonging drugs stack risk. Drugs that lower potassium (some diuretics) can amplify arrhythmia risk. The prescriber and pharmacist need a full list of medications and supplements. Alcohol worsens sedation and orthostasis.

Stopping and tapering

Don't stop thioridazine abruptly. A prescriber can step the dose down gradually.

Pregnancy and breastfeeding

Untreated psychosis carries its own risks in pregnancy, and thioridazine passes into breast milk. Anyone who's pregnant, planning a pregnancy, or breastfeeding should talk it through with their prescriber. This isn't medical advice.

Cost and generic availability

Thioridazine is generic and inexpensive when available. Some pharmacies don't stock it because of low demand, so confirm supply.

Common questions

Why isn't thioridazine used more? QTc prolongation. After post-marketing data linked thioridazine to torsades de pointes and sudden cardiac death, the FDA restricted its use to refractory schizophrenia. Other antipsychotics do similar work with less cardiac risk, so thioridazine dropped off most prescribers' first, second, and third choices.

What is pigmentary retinopathy? A condition where pigment deposits form in the retina, affecting vision. Thioridazine is one of the few psychiatric drugs specifically linked to it, mostly at doses above 800 mg per day. It's a reason for periodic eye exams if the drug is used.

Why does the ECG matter so much? Because thioridazine prolongs the QTc interval more than most antipsychotics, which raises the risk of torsades de pointes. A baseline ECG confirms it's safe to start, and follow-up ECGs catch changes as the dose changes.

Is it safer at low doses? QTc risk is dose-related, so lower doses carry less risk. But even at moderate doses the effect is noticeable, and other antipsychotics generally give similar benefit with less cardiac concern.

Questions to ask your prescriber

• Are we choosing thioridazine because other options have failed?
• How often will we check an ECG?
• Should I have my eyes checked periodically?
• What medications should I avoid while I'm on this?
• If we decide to stop it later, how would we taper safely?

Sources

This guide draws on current prescribing information and public health references. It is reviewed for clinical accuracy and updated as guidance changes, and current as of June 8, 2026.

• U.S. Food and Drug Administration. Thioridazine prescribing information.
• MedlinePlus, U.S. National Library of Medicine. Thioridazine.
• National Institute of Mental Health. Mental health medications.
• American Psychiatric Association. Practice Guideline for the Treatment of Patients with Schizophrenia, 3rd edition.
• National Institute for Health and Care Excellence (NICE). CG178, Psychosis and schizophrenia in adults.
• Carolan A, et al. Metformin for the Prevention of Antipsychotic-Induced Weight Gain: Guideline Development and Consensus Validation. Schizophrenia Bulletin. 2025;51(5):1193 to 1203.