Health tip

Medications That Interfere With Birth Control
by Dr. Rob for MSN Health & Fitness

Q: I am on an estrogen-containing birth control pill to prevent pregnancy. My physician gave me an antibiotic for a urinary tract infection and suggested I use an extra means of contraception during this pill cycle. She said antibiotics make the pill less effective. Is this true?

A: Yes, it’s possible.

In general, drug interactions can enhance, diminish or cause a new effect of the medications involved. For example, using aspirin while you are on the blood thinner warfarin can amplify the effects of this medication, possibly leading to an increased risk of bleeding. On the other hand, taking the antibiotic rifampin can decrease the effectiveness of warfarin, which could raise the risk for blood clots and other problems. Needless to say, whenever you take a medication, it is important to ask your doctor or pharmacist the following questions:

Can I take it at the same time as my other medications?
Does it interact with any foods or drinks? (For example, grapefruit juice.)
Are there any interactions with prescription, nonprescription, herbal or nutritional supplements?
Do I need to take the medication at the same time each day?
What happens if I miss a dose?
What are the potential side effects?
What should you do if a side effect does occur?
Regarding the answer to your question, whether there would be a negative interaction between your medications depends upon what antibiotic your physician prescribed. For example, Rifampin, which is generally used in combination with other drugs to treat tuberculosis, would cause the enzymes in your liver to more quickly break down the estrogen supplied by your birth control pill, rendering your contraceptive less effective. This in turn would increase your chances for an unwanted pregnancy. The antibiotics ampicillin and doxycycline have the potential to interfere with your birth control pill, but to a much lesser extent. Other categories of medications causing a similar interaction with estrogen containing birth control pills include but aren’t limited to:

Phenobarbital (anti-seizure medication)
Griseofulvin (an anti-fungal)
Carbamazepine (anti-seizure)
Topiramate (migraine prevention, anti-seizure)
Even the herbal preparation St. John’s Wort can interfere with the effectiveness of birth control pills, as well as lead to breakthrough bleeding.

As for other antibiotics, the evidence for birth control pill compromise is less clear. In theory, the penicillin and tetracycline category of antibiotics, as well as some others, could impair the effectiveness of the pill. Early studies done 15 to 20 years ago found evidence of such an interaction; however, more recent studies have been inconclusive. This is because there doesn’t appear to be an increased rate of pregnancy when taking common antibiotics while on the birth control pill.

Needless to say, there is a need for better and more up to date information. In the meantime, until better data is obtained, many health-care providers still recommend using an additional method of birth control both during, as well as one week beyond, the last dose of your antibiotic medication.

Finally, it is important to know that even in the best of circumstances the failure rate of oral contraceptives in preventing pregnancy is between 1 percent and 3 percent. That’s why it is especially important to take them properly. Be sure you:

Take them at the same time each day.
Avoid smoking.
Use another form of birth control if you miss several doses during the pill cycle.
Store them at room temperature and away from heat or excess moisture.