Have you ever known someone who became pregnant while taking the Pill, but who swore she took a tablet every day? The culprit could have been a drug interaction.
Certain drugs, notably anticonvulsant medications and some antibiotics, stimulate enzymes which absorb estrogen and progestins. This means less of the hormones from your OCs are available to prevent pregnancy. These drugs can also act on the Norplant system.
If you need to take these medications for only a few weeks, your doctor will probably advise you to use a backup contraceptive, such as condoms or spermicides. Longterm therapy may require you to switch from hormones. Here are some of the medications which can reduce the effectiveness of OCs and implants:
Antibiotics: rifampin, chloramphenicol, cephalosporins, possibly metronidazole, nitrofurantoin.
Anticonvulsants: phenobarbital, primidone, carbamazepine, ethosuximide, phenytoin.
Antifungals: griseofulvin (does not affect Norplant implants).
Source: Outlook, Volume 9, Number 1, April 1991. Program for Appropriate Technology in Health (PATH), Seattle, WA.
