<p>May reduce effectivity of hormonal contraceptives. Reduced absorption with antacids. May decrease plasma concentrations of antivirals (e.g. atazanavir, darunavir, fosamprenavir), atovaquone with rifampicin. Rifampicin may reduce serum levels of anticonvulsants (e.g. phenytoin), antiarrhythmics (e.g. disopyramide), oral anticoagulants, antifungals (e.g. ketoconazole), barbiturates, ?-blockers, Ca channel blockers (e.g. diltiazem), chloramphenicol, clarithromycin, corticosteroids, ciclosporin, cardiac glycosides, clofibrate, dapsone, diazepam, doxycycline, fluoroquinolones (e.g. ciprofloxacin), haloperidol, oral hypoglycemic agents (sulfonylureas), levothyroxine, methadone, narcotic analgesics, progestins, quinine, tacrolimus, theophylline, TCAs (e.g. amitriptyline, nortriptyline) and zidovudine. Increased risk of hepatotoxicity with halothane.<br />
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Isoniazid may inhibit the metabolism of anticonvulsants (e.g. carbamazepine, phenytoin), benzodiazepines (e.g. diazepam), haloperidol, ketoconazole, theophylline, and warfarin. May enhance the CNS effects of meperidine, cycloserine, and disulfiram with isoniazid. Loss of glucose control in patients on oral hypoglycaemics with isoniazid.</p>