Increased breakthrough bleeding incidence w/ rifampicin, INH, ampicillin, tetracycline, neomycin, penicillin, chloramphenicol, sulphonamides, nitrofurantoin, barbiturates, phenytoin, primidone, analgesics, tranquilisers, antimigraine prep, antihistamines. Altered effectiveness of oral anticoagulants, anti-convulsants, TCAs, antihypertensive agents (eg, guanethidine), vit, hypoglycaemic agents, tranquilisers, hypnotic prep & theophylline.