BLACK BOX WARNING
- QT prolongation and sudden cardiac death — reserve only for schizophrenia patients who fail to respond to other antipsychotics
- pigmentary retinopathy with doses >800 mg/day
thioridazine
Brand: Mellaril
⚠ BBW Beers Criteria Prototype: haloperidol
Drug Class: first-generation antipsychotic (FGA)
Drug Family: antipsychotic
Subclass: phenothiazine (piperidine subtype)
Organ Systems: cns
Mechanism of Action
First-generation phenothiazine antipsychotic that blocks D2 dopamine receptors. Also potently blocks IKr potassium channels (highest QT-prolonging risk among antipsychotics), muscarinic receptors (strong anticholinergic), H1, and alpha-1 adrenergic receptors.
D2 dopamine receptorIKr potassium channelmuscarinic receptorshistamine H1 receptoralpha-1 adrenergic receptor
Indications
- schizophrenia refractory to other antipsychotics (last-resort only)
Contraindications
- QT prolongation or known cardiac arrhythmias
- concurrent use of other QT-prolonging drugs
- CNS depression
- severe hypertensive or hypotensive heart disease
- known CYP2D6 poor metabolizers (relative; increases exposure)
Adverse Effects
Common
- QT prolongation (dose-dependent — highest among antipsychotics)
- sedation
- anticholinergic effects (dry mouth, urinary retention, constipation, blurred vision)
- orthostatic hypotension
- weight gain
Serious
- torsades de pointes / sudden cardiac death
- pigmentary retinopathy (dose- and duration-dependent — potentially irreversible blindness)
- neuroleptic malignant syndrome (NMS)
- tardive dyskinesia
- agranulocytosis (rare)
Pharmacokinetics (ADME)
| Absorption | variable oral bioavailability (first-pass effect) |
| Distribution | large Vd; highly lipophilic |
| Metabolism | hepatic CYP2D6 (major); CYP1A2 |
| Excretion | hepatic/biliary; fecal |
| Half-life | 9-30 hours |
| Onset | hours (oral) |
| Peak | 2-4 hours |
| Duration | variable |
| Protein Binding | >99% |
| Vd | large (18-30 L/kg) |
Drug Interactions
| Drug / Agent | Mechanism | Severity |
|---|---|---|
| CYP2D6 inhibitors (fluoxetine, paroxetine, fluvoxamine, propranolol, pindolol) | inhibit thioridazine metabolism — significantly increase plasma levels and QT prolongation risk; contraindicated | contraindicated |
| QT-prolonging drugs | additive QT prolongation and TdP risk | major |
| anticholinergic drugs | additive anticholinergic toxicity | moderate |
| CNS depressants / alcohol | additive CNS and respiratory depression | moderate |
Nursing Considerations
- Baseline and periodic ECGs — QTc >500ms or increase >60ms from baseline warrants discontinuation
- Baseline and annual ophthalmologic exams for retinal pigmentation; patients should report any visual changes
- Monitor for NMS: hyperthermia, muscle rigidity, altered consciousness, autonomic instability — a medical emergency
- Assess for tardive dyskinesia at each visit (AIMS scale) — involuntary orofacial movements
- Orthostatic hypotension: instruct patient to change positions slowly; assess BP lying and standing
- Last-resort agent only — document failure of at least two other antipsychotics before initiating
Clinical Pearls
- Highest QT-prolonging risk among antipsychotics — linked to sudden cardiac death at doses >800 mg/day; largely abandoned in clinical practice
- Pigmentary retinopathy is irreversible and dose/duration dependent — ophthalmology follow-up is mandatory
- CYP2D6 poor metabolizers reach much higher plasma concentrations — combined with inhibitors is contraindicated
- Historically used widely for schizophrenia, now reserved as last resort due to unique toxicity profile
Safety Profile
Pregnancy avoid
Lactation avoid
Renal Adjustment Not required
Hepatic Adjustment Required
TDM Not required
Concordance Terms
Cross-referenced clinical concepts — click any term to see all content where it appears.