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The concomitant use of oestrogens with progestogens may render less effective, and an increased incidence of breakthrough bleeding may occur by virtue of drug interaction with rifampicin, isoniazid, ampicillin, tetracycline, neomycin, penicillin, chloramphenicol, sulphonamides, nitrofurantoin, barbiturates, phenytoin, primidone, analgesics, tranquilisers, antimigraine preparations and antihistamines.
They may also alter the effectiveness of drugs such as oral anticoagulants, anticonvulsants, tricyclic antidepressants, antihypertensive agents (e.g. guanethidine), vitamins, hypoglycaemic agents, tranquilisers, hypnotic preparations and theophylline.
