Drugs Affecting Birth Control Pills

Frederick R. Jelovsek MD

"Do weight-loss pills (xenedrine) effect birth control pills, or cause breakthrough bleeding?". T.B.

Anti-tuberculosis drugs such as rifampin (1), anti-fungal drugs such as griesofulvin and antibiotics such as tetracycline and ampicillin were the most strongly suspected for drug interactions causing an increased pregnancy rate. Dentists, dermatologists and any physician prescribing antibiotics is concerned with whether to advise patients to use extra contraception. Oral contraceptives have also been thought to increase the requirement for anti-epileptic drugs. Many of the anti-seizure medications, with barbituates being the classic drug, stimulate liver enzymes and thus are suspect to speed up the metabolism and degradation of birth control pills.

A large study of over 8058 women taking combined oral contraceptives and seeking elective termination of pregnancy showed 70 women (0.87%) who became pregnant while using oral contraceptives (2). This is about the expected failure rate of 1% for OCPs so it is very unlikely that there is a significant impact of many medicines on oral contraceptive efficacy considering all of the medications that are prescribed.

This seems a natural question because diet pills often speed up a woman's metabolism and you might think that it would speed up the metabolism and degradation of oral contraceptives. Unfortunately there is no data in the medical scientific literature one way or the other. It simply has not been studied in any clinical trials. As with most other drug interactions with OCPs it is unlikely that diet pills would decrease the effectiveness of birth control pills but this is just an educated guess.

Early data about 20 years ago seemed to indicate that when antibiotics were taken along with birth control pills, more women got pregnant than you would normally expect. Drugs like ampicillin and tetracycline were suspected to interfere with OCPs (3). However, all of the recent studies that have looked at this, have shown that antibiotics do not increase the pregnancy rate at all (4) and they point out that the old data was not reliable enough to draw conclusions about pregnancy rates on any of the antibiotics (5).

Some antibiotics have been studied and shown not to affect the metabolism of OCPs. Ciprofloxcin (Cipro ®) is one that does not seem to alter metabolism (6). Fluconazole (Diflucan ®) does not decrease estrogen levels in pill users; if anything, it raises estrogen levels (7).

There are some older studies showing a higher pregnancy rate among epileptic women on anti-seizure medications and taking oral contraceptives (8). Also, teens have a higher abnormal bleeding rate when on the combination of OCPs and anti-epileptic drugs and this goes away when the estrogen levels in the pills are increased (9). In general, it is felt that certain anti epileptic drugs stimulate or induce liver enzymes to metabolize birth control pills faster but there is not good clinical data to make recommendations for practice guidelines such as to increase the dose of pills to prevent pregnancy (10, 11). In practice, however, most experts recommend using higher dose birth control pills for women who are on anti-epileptic drugs or at least avoiding the very low dose pills.

Folate levels also tend to be lower while on anti-epileptic drugs. Taking birth control pills adds to this lowering (12) so that folate supplementation is recommended.

The most common question situation that comes up is with a flu, gastritis or diarrhea condition and whether that affects absorption of the contraceptive pill. There is very little data to support this concept at all. It may be because episodes of acute gastrointestinal illness do not last long. We know that missing up to 10 pills in a row (7 placebo pills and the 1st 3 pills of a pill pack) does not result in ovulation (13). Therefore acute illnesses are unlikely to result in a decreased efficacy of birth control pills. This is probably why most antibiotics don't actually cause unwanted pregnancies because they are given in regimens of usually 10 days or less. Long term medications that are suspected of inducing liver enzymes to a large extent should be treated as if they may require higher oral contraceptive doses.