Enzyme inhibition leads to the slow metabolism of the drugs. This causes greater serum concentration and toxicity. An example is Ethinyl estradiol (an active constituent in birth control pills) with grape fruit juice. Grape fruit juice is a drug inhibitor of CYP3A4 Isoezyme. With this interaction grapefruit juice tends to increase the incidence of adverse reactions in Ethinyl estradiol as gastric upsets or headaches.
On the other hand, enzyme inducers hasten the biodegration of drugs. With this, efficacy of the medicine is decreased. The rifampicin (drug for tuberculosis) for instance, will lessen the effect of a contraceptive pill.