" 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.
Do antibiotics make birth control pills less
effective?
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).
Do birth control pills cause more seizures
in women being treated for epilepsy?
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.
Are there other diseases or conditions that
can decrease the effectiveness of birth control pills?
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.
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