Drugs Affecting Birth Control Pills
Frederick R. Jelovsek MD
"Do weight-loss pills (xenedrine) effect birth control pills, or cause breakthrough bleeding?". T.B.
Drug-drug interactions with oral contraceptives that may reduce their efficacy have long been worrisome but not always well documented.
It was felt on a mechanistic basis that any drug which stimulated liver enzyme production could theoretically lessen the effectiveness of birth control pills because they are mainly metabolized in the liver.
While there were anecdotal reports of women becoming pregnant while on a specific drug, there were very few studies that looked critically at whether women taking oral contraceptives and on the drug had any more unintended pregnancies than women not taking the drug.
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.
Is there an interaction between diet pills and oral contraceptive efficacy?
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.