Mesoridazine, marketed as Serentil by Boehringer Ingelheim and later by Novartis, is one of the most thoroughly retired antipsychotics in the modern pharmacopoeia. It was introduced in the United States in 1970, used through the 1970s, 1980s, and 1990s, and discontinued in the US in 2004 after a black box warning about QT prolongation made it commercially unviable. It is included here for historical completeness — patients sometimes ask about old medications they were on, and clinicians may encounter mesoridazine in old records.
Mesoridazine is a phenothiazine antipsychotic and the active metabolite of thioridazine, withdrawn from the US market in 2004 because of dose-related QT prolongation and sudden cardiac death risk.
Where it came from
When researchers studied how thioridazine is metabolised in the body, they found that one of its main active metabolites — mesoridazine — was itself a potent antipsychotic. Mesoridazine was developed and approved as a separate product, on the reasoning that giving the active metabolite directly might offer better predictability of effect. The pharmacology turned out to be very similar to the parent drug, including the cardiac liability.
How it worked
Mesoridazine blocks dopamine D2 receptors and has substantial affinity for histamine H1 (sedation), muscarinic acetylcholine (anticholinergic effects), and alpha-1 adrenergic (orthostatic hypotension) receptors. It is a low-potency phenothiazine, with adult doses typically in the range of 25 to 200 mg per day. Like its parent thioridazine, it strongly blocks the cardiac hERG potassium channel, which is the molecular basis of its QT-prolonging effect.
Why it was withdrawn
By the late 1990s, accumulating evidence showed that mesoridazine, like thioridazine, prolonged the QT interval on ECG in a dose-dependent way and was associated with sudden cardiac death. The FDA required a black box warning in 2000, restricting use to schizophrenia patients who had failed other antipsychotic treatment and requiring ECG monitoring. By 2004, the manufacturer had ceased US production. The drug's removal was a commercial decision driven by a small market and increasing safety concerns.
Do not take any antipsychotic that has been sitting in a medicine cabinet for years. Speak with a prescriber about whether the drug is still appropriate, whether it is still available, and what the modern alternatives are.
Side effects when it was in use
Aside from the cardiac issue, mesoridazine had the typical low-potency phenothiazine profile:
- Sedation, often substantial
- Dry mouth, constipation, urinary retention, blurred vision
- Orthostatic hypotension and falls
- Modest extrapyramidal symptoms (less than high-potency typicals)
- Tardive dyskinesia with long-term use
- Sexual dysfunction including ejaculatory disturbance
- Weight gain
- Hyperprolactinemia
- Photosensitivity
- Rare hepatic and haematologic effects
What patients on mesoridazine should do today
Anyone still on mesoridazine — which would be unusual but not impossible in some international settings — should discuss with a prescriber whether to continue. ECG monitoring would be essential. In high-income settings, the standard recommendation is to switch to a modern antipsychotic with a better-characterised cardiac safety profile. Sudden discontinuation is not advised; cross-titration with a prescriber's guidance is the safer route.
Why this story matters
Mesoridazine's withdrawal — like thioridazine's restriction — illustrates how long it can take for safety signals to emerge from clinical use. Both drugs were prescribed widely for decades before the cardiac risk was characterised well enough to change practice. The development of routine ECG monitoring, post-marketing surveillance, and the FDA's QT-interval guidance in the early 2000s shifted the standard of care in psychopharmacology. Several other antipsychotics — including high-dose haloperidol, droperidol, and pimozide — were re-examined in the same period.
What replaced it
Patients who would historically have been prescribed mesoridazine are now treated with second-generation antipsychotics or with other first-generation agents that have a more favourable cardiac profile (haloperidol or perphenazine, for example, although both can also prolong QT at higher doses). The general modern preference is to start with an atypical agent and reserve typicals for specific situations.
Practical questions to ask a prescriber
- Why was I (or my relative) on mesoridazine historically?
- What modern medication would be closest in effect?
- Is there anything specific from that history I should know about (cardiac risk, EPS history)?
- What baseline ECG and labs do I need before starting a new antipsychotic?
The big picture
Mesoridazine is part of the history of psychopharmacology rather than its present. Its story is a useful reminder that "approved by the FDA" does not mean "permanently safe" — long-term surveillance, careful comparisons with newer agents, and willingness to retire drugs when better options exist are all essential parts of how the field improves over time.
This article is for educational purposes only and is not medical advice. Information is summarised from publicly available FDA labelling, regulatory sources, and peer-reviewed literature. Always consult your prescribing clinician before starting, stopping, or changing any medication.