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Information For Patients: Antibiotic And Oral Contraceptive Interactions
There has been a lot of talk about whether taking dicloxacillin (Dynapen, etc.), tetracycline (Sumycin, antibiotics can interfere with birth control pills and etc.), doxycycline (V i b r a m y c i n , etc.), and lead to pregnancy. The overall risk is small and minocycline (Minocin, etc.). Keep in mind that this applies more to some antibiotics than others. Here is not a complete list and other antibiotics might is the current thinking of what experts believe is also decrease the effectiveness of your birth control pill. Check with your pharmacist or prescriber ifyou need to take any antibiotic and are concerned Will My Antibiotics Interact With My Birth
Control Pills?
The actual incidence of pregnancy in women What Can I Do To Prevent Pregnancy While On
who take antibiotics while on birth control pills is Antibiotics?
unknown. There have been few reports considering Even though the risk of getting pregnant when the huge number of women who take birth control taking antibiotics while on birth control pills is pills each year. However, there is a risk. For small, there is still a risk. Remember that it’s example, the antibiotic rifampin will interact with usually not possible tell whether or not you are one your pill, and other antibiotics might also interact.
of the women who are at high risk. This is Some women may be more susceptible than others especially important today because we now have to getting pregnant when taking both antibiotics and birth control pills with lower doses of hormones to birth control pills. Unfortunately, it’s not possible reduce side effects of the pill. If you take a low- to predict which women are at higher risk.
dose birth control pill you might be at higher risk ofgetting pregnant while taking antibiotics. Low- Why Does An Interaction Sometimes Occur?
dose birth control pills include: Alesse, Aviane, Enzymes in the liver metabolize many of the Cyclessa, Levlite, Loestrin 1/20, Loestrin Fe 1/20, drugs we take, including the hormones in birth control pills. Some antibiotics are metabolized by If you need to take rifampin or griseofulvin, and the same enzyme as the hormones in birth control want to continue taking birth control pills, you pills, and can cause the hormones to be metabolized should also use a second method of birth control faster. When this happens, the blood levels of the while on these antibiotics. Another option is to hormones decrease, and the birth control pill change to a high-dose birth control pill. Check with becomes less effective. These antibiotics are very your prescriber about what is the best choice for likely to make your birth control pill less effective.
Other antibiotics can decrease the bacteria If you need to take any other type of antibiotic normally found in the intestines of healthy people.
and are not comfortable with the risk of getting pregnant you should also use a second method of compounds and release active hormone which is birth control. You should also use a second method then reabsorbed. Without these bacteria, there’s of birth control if you develop breakthrough less active hormone in the bloodstream for the body bleeding while on antibiotics or if you’ve gotten to use. Lower hormone levels can increase the pregnant while on birth control pills in the past.
chance that your birth control pill won’t work.
The second method should be a nonhormonal type Which Antibiotics?
(condoms, abstinence, etc.). If you will be taking Some antibiotics are more likely than others to antibiotics SHORT-TERM, you should also use the make your birth control pill less effective.
second method of birth control for the full time you Antibiotics that speed up the metabolism of the pill, are taking the antibiotics, and for at least 7 days such as rifampin (Rifadin, etc.) and griseofulvin after you finish them or until the end of your cycle.
(Gris-PEG, etc.), are the most likely to interact.
If you will be taking antibiotics LONG-TERM, Other antibiotics, such as those in the penicillin and such as for acne, you should also use the second tetracycline classes, also carry a risk of making method of birth control for at least two weeks after your birth control pill less effective, although the starting the antibiotic, until your intestinal bacteria risk is less than with rifampin. Some antibiotics in become resitant, and the birth control pill becomes the penicillin and tetracycline classes include effective again. Check with your pharmacist or penicillin (Pen Vee K, etc.), ampicillin (Omnipen, prescriber if you have any questions about how to etc.), amoxicillin (A m o x i l , Augmentin, etc.), The most practical knowledge in the least time… 3120 West March Lane, P.O. Box 8190, Stockton, CA 95208 Y TEL (209) 472-2240 Y FAX (209) 472-2249 —This Detail-Document accompanies the related article published in— PHARMACIST’S LETTER / PRESCRIBER’S LETTER
February 2002 ~ Volume 18 ~ Number 180214 Antibiotic-Oral Contraceptive Drug Interactions
Lead author: Kay Shaver, Pharm.D., Assistant Editor Frequency Of Occurrence
SHBG’s binding capacity can decrease the levels It is well-known that certain drugs, such as phenobarbital, phenytoin, and carbamazepine, Antibiotics that don’t affect liver metabolism interact with oral contraceptives (OCs) and can can interfere with OC efficacy by another increase the risk of contraceptive failure and mechanism. Intestinal bacteria break down pregnancy. But evidence supporting antibiotics’ estrogen conjugates to restore free estrogen for role in decreasing the efficacy of OCs is less reabsorption via enterohepatic circulation. Some definitive, and so this drug interaction is antibiotics may alter intestinal bacteria which somewhat controversial. The information we interrupts enterohepatic circulation and can lead to have generally comes from case reports, adverse- a decrease in the amount of circulating estrogen.
event monitoring systems, and retrospective Interestingly, some antibiotics may actually surveys, rather than controlled clinical trials.1,2 increase the levels of ethinyl estradiol and some There are relatively few reported cases of oral progestins. Macrolides and azole antifungals contraceptive failure in women who also took inhibit the same liver enzyme (CYP450 3A4) that antibiotics, especially considering the large number of women who use OCs. Surveys have It’s possible that certain women are at high risk shown failure rates of 1.2% to 1.6% in women for an interaction between antibiotics and OCs.
taking both OCs and antibiotics. Although these The actual incidence of OC failure is not known, numbers are greater than those ideally predicted and unfortunately, the risk of failure can’t be with perfect compliance, they are within the 1% to predicted in individual women. There are large 3% that is seen with typical use. Because of this low rate of OC failure, it is difficult to determine concentration and clearance of estrogens and whether such an incidence is due to the antibiotic progestins. This means that some women might or is within the expected failure rate.1,2 be more susceptible to OC failure while receivingantibiotics, but we are not able to predict which Mechanism
women. This is especially important consideringthe development of new, lower-dose oral Ethinyl estradiol is the estrogenic component contraceptives (Cyclessa, Levlite, Mircette, etc.).
of most combined oral contraceptives. It is metabolized in the liver by cytochrome P450 3A4, effectiveness of OCs may be more likely when a excreted in the bile, broken down by intestinal woman is taking lower-dose contraceptives.1 bacteria, and then reabsorbed as an active drug.1,3,4 The chance of having a significant interaction between OCs and antibiotics depends on several antibiotics can affect OC effectiveness involve liver metabolism and enterohepatic circulation.
estrogen/progestin in the OC and the amount The most significant interactions are those that needed by the woman, 2) dose and duration of use occur with other drugs metabolized by the same of the antibiotic, 3) individual variation in liver enzyme as estrogen. These would include response to alteration of bacterial flora, and 4) rifampin and griseofulvin. Rifampin is a potent fertility of the couple. Predicting the outcome in inducer of cytochrome P450 3A4 and accelerates specific women is difficult if not impossible.3 the elimination of ethinyl estradiol and someprogestin components, resulting in an increasedrisk of OC failure. Rifampin also affects sex Recommendations
hormone-binding globulin (SHBG). SHBG has a Today it is common to counsel women taking high affinity for progestins and drugs that increase broad-spectrum antibiotics that treatment with Copyright  2002 by Therapeutic Research Center Pharmacist’s Letter / Prescriber’s Letter • P.O. Box 8190, Stockton, CA 95208 • Phone: 209-472-2240 • Fax: 209-472-2249 these drugs may reduce the efficacy of their OC.
small overall risk of a drug interaction with Although the risk of OC failure appears to be very antibiotics, and that it’s not possible to identify in advance who is at high risk for OC failure. Tell pregnancy should be taken very seriously. In women who aren’t comfortable with this risk to addition, there is not yet enough evidence to also use a nonhormonal method of contraception.
support abandoning this practice. It is advisable Women who have had OC failures in the past or to be cautious in order to protect those women who develop breakthrough bleeding while using using OCs who may be at high risk of OC failure antibiotics with OCs should use an alternate method of contraception. These women may be part of the group of women at high risk of significant risk of OC failure. A different method of contraception can be suggested if a woman willbe taking rifampin or griseofulvin long-term. For Users of this document are cautioned to use their own women who choose to use OCs while taking these professional judgment and consult any other necessary or appropriate sources prior to making clinical nonhormonal method of contraception, such as judgments based on the content of this document. Our condoms or abstinence, during treatment.
editors have thoroughly researched the information Another alternative is to switch to a high-dose (50 with input from experts, government agencies, and micrograms of estrogen) pill during treatment.
national organizations. Information and Internet links Suspect interference with OC effectiveness if in this article were current as of the date of breakthrough bleeding and/or spotting occurs while taking these drugs together with OCs.1,3,5 Switching to a pill containing a high-dose of References
estrogen is not practical during typical short-term 1. Dickinson BD, Altman RD, Nielsen NH, et al. Drug use (less than two weeks) of antibiotics. Instead, interactions between oral contraceptives andantibiotics. Obstet Gynecol 2001;98:853-60.
counsel women to continue taking their OC, but to 2. Ament PW, Bertolino JG, Liszewski JL. Clinically also use a back-up method of contraception such significant drug interactions. Am Fam Physician recommend continuing to use both methods of 3. Ruggiero RJ. Contraception. In: Young LY, Koda- contraception for the duration of antibiotic treatment and for at least seven days thereafter.
Therapeutics: The Clinical Use of Drugs . 6th ed.
Other sources recommend the alternate method of Vancouver, WA: Applied Therapeutics, Inc., contraception should be continued for the duration of the cycle. A different approach can be used 4. Carmichael JM, Wieland KA. Contraception and during long courses of antibiotics, for example infertility. In: Herfindal ET, Gourley DR, eds.
when tetracycline is used for acne. In this case, Textbook of Therapeutics: Drug and DiseaseManagement. 6th ed. Baltimore, MD: Williams & some experts recommend using a back-up method of contraception for at least two weeks from the 5. International Planned Parenthood Federation.
start of antibiotic treatment, until the intestinal Statement on steroidal oral contraception.
flora become resistant, to allow restoration of September 1998. Available at: www.ippf.org.
Counsel women who use OCs that there is a The most practical knowledge in the least time… 3120 West March Lane, P.O. Box 8190, Stockton, CA 95208 Y TEL (209) 472-2240 Y FAX (209) 472-2249 Copyright  2002 by Therapeutic Research Center Subscribers to Pharmacist’s Letter and Prescriber’s Letter can get Detail-Documents, like this one, on any topic covered in any issue by going to www.pharmacistsletter.com or www.prescribersletter.com

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