Haloperidol + Promethazine + Diazepam ("Cocktail") in a Warfarin Patient

1. Haloperidol + Warfarin

• Haloperidol is primarily metabolized by CYP2D6 (and to a lesser extent CYP3A4).
• Warfarin is primarily metabolized by CYP2C9, with minor contributions from CYP1A2 and CYP3A4.
• The two drugs do not share the same primary metabolic pathway, so direct PK competition is not the main concern.
• However, haloperidol is a known CYP2D6 substrate (not an inhibitor of CYP2C9), so it does not typically elevate INR through enzyme inhibition of warfarin's primary pathway.

• Haloperidol can cause QT prolongation. Warfarin itself does not prolong the QT, but if the underlying reason the patient is on warfarin is atrial fibrillation or a cardiac condition, monitoring is prudent.
• Some older case reports suggest haloperidol may occasionally potentiate anticoagulant effect, though the mechanism is unclear.

2. Promethazine (Phenergan) + Warfarin

• Promethazine is metabolized by CYP2D6 and CYP2B6 - again, not primarily CYP2C9.
• It has some CYP2D6 inhibitory activity, which can raise levels of other CYP2D6 substrates (e.g., haloperidol), potentially increasing haloperidol toxicity rather than warfarin toxicity.
• Direct interaction with warfarin is not well-established - no formal listing as a major warfarin interactor in standard references.

• Additive CNS depression when combined with diazepam and haloperidol.
• Anticholinergic effects.

3. Diazepam + Warfarin

• Diazepam is a CYP3A4 substrate (also CYP2C19).
• When used with CYP3A4 inhibitors (e.g., omeprazole), diazepam levels may rise. Importantly, diazepam itself does not significantly inhibit CYP2C9, so it has limited direct impact on warfarin metabolism.
• Some references note that omeprazole can raise both diazepam and warfarin levels simultaneously - but this is via omeprazole, not diazepam directly.
• Benzodiazepines as a class are not listed as major warfarin interactors in standard guidelines (Tintinalli's, UC San Diego anticoagulation guidelines, etc.).

4. The Triple Combination ("Cocktail") - Cumulative Considerations

Key Risks to Watch in This Scenario

1. CNS/Respiratory depression - All three drugs are CNS depressants. In combination, the sedative effect is heavily additive/synergistic. In an elderly or frail patient on warfarin (who may already be sick), this raises fall risk and risk of aspiration - indirectly increasing hemorrhage risk from a fall.
2. INR monitoring - While none of these drugs are major warfarin interactors, it is good practice to check INR 3-5 days after initiating any new medication in a warfarin patient.
3. Underlying indication matters - If the patient is on warfarin for atrial fibrillation, haloperidol's QT-prolonging effect needs separate attention.
4. Pharmacogenomics - If the patient is a poor CYP2D6 metabolizer (or is on a CYP2D6 inhibitor like fluoxetine/paroxetine), both haloperidol and promethazine levels may accumulate, causing more CNS/extrapyramidal toxicity without INR impact.

Bottom Line

• INR is monitored (check at baseline and 3-5 days after administration)
• Adequate respiratory monitoring is in place given triple CNS depression
• Dose adjustments are made for renal/hepatic impairment, which can affect both drug levels and warfarin sensitivity
• Fall precautions are in place given sedation risk

Sources: Kaplan & Sadock's Comprehensive Textbook of Psychiatry (antipsychotic CYP metabolism table); Tintinalli's Emergency Medicine (warfarin drug interactions); Katzung's Basic and Clinical Pharmacology 16e; Drug interactions with warfarin and psychotropics (Pharmacotherapy); Drug Interactions Affecting Oral Anticoagulant Use - PMC/NIH.