Penicillin was found to have no association with the efficacy of OCP when used together. It is excreted through kidney. Drugs which induce Cyt P450 was found to lower the efficacy of OCPs when combined together. Drugs which induce Cyt P450 are: Rifampin Griseofulvin Phenytoin, mephenytoin Phenobarbital Primidone Carbamazepine Ethosuximide Ref: Cunningham F.G., Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 32. Contraception.