Does penicillin or ampicillin interfere with birth control pills? And if so, how long must one use alternative forms of birth control after taking these medications?

— Cross-purposes

Dear Cross-purposes,

It is common to hear that antibiotics, including penicillin and ampicillin, have the potential to interfere with the effectiveness of birth control pills; however the only antibiotic shown to decrease effectiveness of the pill in studies is rifampin. Luckily, rifampin isn't commonly prescribed in the United States (it is usually used to treat tuberculosis).

Researchers are continuing to investigate the matter, however, it is not necessary to use a back up method of birth control while on antibiotics. This is because antibiotics don't lower the levels of hormones in your blood to the point that you would be at risk for pregnancy. The exception would be if you are taking rifampin (as noted above) or if your doctor has diagnosed an underlying medical condition that interferes with absorption.

Now, if you are vomiting and unable to keep food or pills down because of your illness, you may end up missing your daily dose of birth control. Whether or not you're on antibiotics, if you miss one of your pills due to vomiting (or if you miss a pill for any other reason), you'll want to call your health care provider to get their recommendation as to whether and for how long you need back up.

Here are some other guidelines to keep in mind when using antibiotics:

  • Always take the entire dosage prescribed by your doctor. This means that if you are given a seven day prescription and you feel better after two days, continue to take the antibiotics for the full seven days to insure that the infection is cleared.
  • Avoid alcohol when you are on antibiotics, as there is an increased possibility of getting physically ill with this combination.
  • Talk to your doctor about any possible side effects, and monitor your symptoms carefully.

There are other medications, including certain medications used to treat HIV, prevent seizures, and treat fungal infections, and the herbal supplement St. John's wort, that may decrease the effectiveness of hormonal birth control methods. Additionally, analgesics (like Tylenol® and Excedrin®) and anti-hypertensivesmay be less effective if hormonal birth control methods are also used. So, it's always a good idea to make your health care provider aware of the medications (both prescription and over-the-counter ones) you are currently taking. If you're being prescribed a new medication, check with a health care provider or pharmacist if you're unsure about how medications may interact or have general concerns about birth control methods. Take care!


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