methadone and phenytoin

Phenytoin is a potent inducer of cytochrome P450 enzymes, particularly CYP3A4 and CYP2C19. Methadone is extensively metabolised by CYP3A4, CYP2B6, and CYP2C19. Co-administration of phenytoin increases the hepatic clearance of methadone, leading to reduced plasma methadone concentrations.

The reduced methadone exposure can precipitate opioid withdrawal symptoms, including anxiety, agitation, nausea, vomiting, diarrhoea, abdominal cramps, and drug craving. This may lead to poor treatment outcomes in patients receiving methadone for opioid dependence or chronic pain. In some cases, withdrawal symptoms can be severe and may undermine the therapeutic goals.

Avoid concurrent use if possible. If co-prescription is necessary, closely monitor the patient for signs of opioid withdrawal. Consider increasing the methadone dosage, with careful titration based on clinical response. Monitor plasma methadone concentrations where available, particularly after initiating or discontinuing phenytoin. Re-evaluate the methadone dose when phenytoin is stopped, as methadone levels may rise and cause toxicity. Involve the patient and relevant specialists in dose adjustments.