Interaction between Phenobarbital and Norgestrel
Major Metabolism

ID DDInter1442 and DDInter1315
Interaction RECOMMENDED: Coadministration with certain anticonvulsants such as carbamazepine, eslicarbazepine, oxcarbazepine, phenobarbital, phenytoin, and primidone may reduce the efficacy of contraceptive hormones. There have been numerous case reports of menstrual abnormalities (e.g., breakthrough bleeding, amenorrhea, irregular menses) and unintended pregnancy occurring in women who received oral contraceptives with anticonvulsants. The interaction stems from accelerated clearance of contraceptive hormones as well as decreased plasma concentrations of unbound (active) hormones due to induction of hepatic CYP450 enzymatic activity and hormone-binding globulin capacity by some anticonvulsants.
Management Women using hormonal contraceptives should be advised of the risk of breakthrough bleeding and unintended pregnancy during concomitant therapy with enzyme-inducing anticonvulsants.
References
Alternative for Phenobarbital N05C
Methohexital
Quazepam
Triazolam
Nitrazepam
Paraldehyde
Chloral hydrate
Propiomazine
Scopolamine (ophthalmic)
Midazolam
Estazolam
Dichloralphenazone
More
Alternative for Norgestrel G03F
Desogestrel

G03A
Desogestrel
Norelgestromin

Potential Metabolism Interactions

Substrate-Substrate Interaction：If more than one drug is metabolized by the same CYP, it is possible that its metabolism is inhibited because of the competition between the drugs. That means, it can be useful to lower the dosage of the drugs in the drug-cocktail because they remain longer in the organism than in monotherapy.
Inhibitor-Inhibitor Interaction：Combining two or more inhibitors of one CYP, should be compensated by lowering the dosage of these drugs because the metabolism is reduced and the drugs remain longer in the organism than in monotherapy. Not adapting the dosage bears the risk of even more side effects.
Inhibitor-Substrate Interaction：Combining drugs that have inhibitory effect and are substrates of one particular CYP, should be compensated by lowering the dosage. They rest longer in the organism than in monotherapy. Not adapting the dosage bears the risk of even more side effects.