Drug interactions which result in an increased clearance of sex hormones can lead to breakthrough bleeding and oral contraceptive failure. This has been established with hydantoins, barbiturates, primidone, carbamazepine and rifampicin; oxcarbazepine, topiramate, felbamate and griseofulvin are also suspected. The mechanism of this interaction appears to be based on the hepatic enzyme-inducing properties of these drugs. Maximal enzyme induction is generally not seen for 2-3 weeks but may then be sustained for at least 4 weeks after the cessation of drug therapy.
Contraceptive failures have also been reported with antibiotics eg, ampicillins and tetracyclines. The mechanism of this effect has not been elucidated. Women on short-term treatment with any of the previously mentioned classes of drugs or individual drugs should temporarily use a barrier method in addition to the COC ie, during the time of concomitant drug administration and for 7 days after their discontinuation. For women on rifampicin, a barrier method should be used in addition to the COC during the time of rifampicin administration and for 28 days after its discontinuation. If concomitant drug administration runs beyond the end of the tablets in the COC pack, the next COC pack should be started without the usual tablet-free interval.
In women on long-term treatment with hepatic enzyme-inducing drugs, experts have recommended to increase the contraceptive steroid doses. If a high contraceptive dosage is not desirable or appears to be unsatisfactory or unreliable eg, in the case of irregular bleeding, another method of contraception should be advised.
Laboratory Tests: The use of contraceptive steroids may influence the results of certain laboratory tests, including biochemical parameters of liver, thyroid, adrenal and renal function, plasma levels of (carrier) proteins eg, CBG and lipid/lipoprotein fractions, parameters of carbohydrate metabolism and parameters of coagulation and fibrinolysis. Changes generally remain within the normal laboratory range.