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Information For Patients: Antibiotic And Oral Contraceptive Interactions
(Prescriber's Letter 2002:9(2):180214)


There has been a lot of talk about whether taking antibiotics can interfere with birth control pills and lead to pregnancy. The overall risk is small and applies more to some antibiotics than others. Here is the current thinking of what experts believe is happening:

Will My Antibiotics Interact With My Birth Control Pills?
The actual incidence of pregnancy in women who take antibiotics while on birth control pills is unknown. There have been few reports considering the huge number of women who take birth control pills each year. However, there is a risk. For example, the antibiotic rifampin will interact with your pill, and other antibiotics might also interact. Some women may be more susceptible than others to getting pregnant when taking both antibiotics and birth control pills. Unfortunately, it's not possible to predict which women are at higher risk.

Why Does An Interaction Sometimes Occur?
Enzymes in the liver metabolize many of the drugs we take, including the hormones in birth control pills. Some antibiotics are metabolized by the same enzyme as the hormones in birth control pills, and can cause the hormones to be metabolized faster. When this happens, the blood levels of the hormones decrease, and the birth control pill becomes less effective. These antibiotics are very likely to make your birth control pill less effective.

Other antibiotics can decrease the bacteria normally found in the intestines of healthy people. These bacteria help break down estrogen compounds and release active hormone that is then reabsorbed. Without these bacteria, there's less active hormone in the bloodstream for the body to use. Lower hormone levels can increase the chance that your birth control pill won't work.

Which Antibiotics?
Some antibiotics are more likely than others to make your birth control pill less effective. Antibiotics that speed up the metabolism of the pill, such as rifampin and griseofulvin (Gris-PEG, etc.), are the most likely to interact.

Other antibiotics, such as those in the penicillin and tetracycline classes, also carry a risk of making your birth control pill less effective, although the risk is less than with rifampin. Some antibiotics in the penicillin and tetracycline classes include penicillin (Pen Vee K, etc.), ampicillin, amoxicillin (Amoxil, Augmentin, etc.), dicloxacillin ,tetracycline, doxycycline and minocycline.

Keep in mind that this is not a complete list and other antibiotics might also decrease the effectiveness of your birth control pill. Check with your pharmacist or prescriber if you need to take any antibiotic and are concerned about the risk of getting pregnant.

What Can I Do To Prevent Pregnancy While On Antibiotics?
Even though the risk of getting pregnant when taking antibiotics while on birth control pills is small, there is still a risk. Remember that itís usually not possible to tell whether or not you are one of the women who are at high risk. This is especially important today because we now have many birth control pills with lower doses of hormones to reduce side effects of the pill.

If you take a low-dose birth control pill you might be at higher risk of getting pregnant while taking antibiotics. Low-dose birth control pills include: Alesse, Aviane, Cyclessa, Levlite, Loestrin 1/20, Loestrin Fe 1/20, Mircette, and others.

If you need to take rifampin or griseofulvin, and want to continue taking birth control pills, you should also use a second method of birth control while on these antibiotics. Another option is to change to a high-dose birth control pill. Check with your prescriber about what is the best choice for you.

If you need to take any other type of antibiotic and are not comfortable with the risk of getting pregnant you should use a second method of birth control. You should also use a second method of birth control if you develop breakthrough bleeding while on antibiotics or if you've gotten pregnant while on birth control pills in the past. The second method should be a nonhormonal type (condoms, abstinence, etc.).

If you will be taking antibiotics SHORT-TERM, such as for an infection, you should use the second method of birth control for the full time you are taking the antibiotics, and for at least 7 days after you finish them.

If you will be taking antibiotics LONG-TERM, such as for acne, you should use the second method of birth control for at least two weeks after starting the antibiotic.

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