Phenobarbital and other barbiturates may reduce the activity of many drugs by increasing the rate of metabolism through induction of drug-metabolising enzymes in liver microsomes. Drugs which may be affected include carbamazepine, coumarin anticoagulants, cyclosporin, doxycycline, phenylbutazone, metronidazole, phenytoin, quinidine, theophylline, tricyclic antidepressants and corticosteroids and other steroid hormones including oral contraceptives. Phenobarbital may also reduce the activity of griseofulvin although the mechanism is not entirely clear.
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