- Use of drugs that may reduce the effectiveness of COCP from the start of the Marvelon cycle until last pharmacokinetic sample. For example, phenytoin, barbiturates, primidone, carbamazapine, oxcarbazepine, topiramate, felbamate, rifampicin, nelfinavir, ritonavir, griseofulvin, oral ketoconazole, and herbal remedies containing Hypericum perforatum (St John's Wort)