May alter effect of warfarin. Tramadol: Additive effects w/ other opioids (eg, fentanyl, pethidine) or illicit drugs causing CNS depression (eg, γ-hydroxybutyrate, flunitrazepam). Decreased analgesic effect & risk of tramadol-associated seizures w/ carbamazepine. Decreased plasma conc w/ CYP3A4 inducers eg, rifampicin, St. John's wort. Reduced metabolic clearance w/ CYP2D6 inhibitors (eg, amitriptyline, fluoxetine, paroxetine) &/or CYP3A4 inhibitors (eg, ketoconazole, erythromycin). Increased plasma conc w/ quinidine. Increased risk of serious adverse events including seizures & serotonin syndrome w/ serotonergic drugs (MAOIs, SSRIs, SNRIs, triptans, lithium). Paracetamol: Increased risk of hepatotoxicity w/ anticonvulsants eg, phenytoin, barbiturates, carbamazepine. Possible increased risk of hepatotoxicity w/ INH & severe hypothermia w/ phenothiazines. Additive effects w/ ethanol.