<p>Enzyme-inducing drugs may result in increased clearance and lead to breakthrough bleeding and contraceptive failure. This may be seen with hydantoins, barbiturates, primidone, carbamazepine, rifampicin, oxcarbazepine, rifabutin, felbamate, ritonavir, griseofulvin and products containing St John&#39;s Wort. Reduced absorption of etonogestrel may be seen with medical charcoal.</p>