Amoxicillin and Birth Control Pills: What You Need to Know

The old concern rests on one proposed mechanism. Amoxicillin, a penicillin-type antibiotic, changes the bacteria living in your gut, and those bacteria help convert estrogen back into a form the body can reabsorb after it has passed through the liver. The FDA label for amoxicillin still notes that it may affect the intestinal flora, leading to lower estrogen reabsorption and reduced efficacy of combined oral estrogen/progesterone contraceptives (droracle.ai, citing FDA drug labeling).

When researchers measured actual hormone levels instead of reasoning from theory, that effect largely did not appear. A review in the American Journal of Obstetrics & Gynecology found that evidence from clinical and pharmacokinetic outcomes studies does not support the existence of drug interactions between hormonal contraception and non-rifamycin antibiotics, with no significant decreases in any progestin measurement during co-administration with any antibiotic (National Library of Medicine, January 2018). Earlier pharmacokinetic work found plasma levels of contraceptive steroids unchanged alongside ampicillin, doxycycline, metronidazole and tetracycline, among others, and one comparison found five pregnancies in 311 woman-years of combined antibiotic and oral contraceptive exposure against 12 pregnancies in 1,245 woman-years among controls — a nonsignificant difference (National Library of Medicine).

The genuine exception works through a different route. Rifampin, rifabutin and griseofulvin speed up the liver's clearance of contraceptive hormones rather than acting on gut bacteria; rifampin increased the frequency of ovulation in two of four studies and reduced estrogen or progestin exposure in five (CDC). Amoxicillin is not in that group.

Two caveats remain. Study authors note that a small subset of women may still be at risk for hormonal contraceptive failure while taking antibiotics (The ObG Project), and there is no test that identifies who. And nausea, vomiting and diarrhea are the most common amoxicillin side effects (GoodRx) — a pill brought back up or hurried through the gut may not be absorbed.

Report any of these to a provider rather than deciding what they mean yourself. They run from mild to urgent:

• Nausea, mild diarrhea or a queasy stomach after doses — common with amoxicillin, but tell your pharmacist if you vomit within a few hours of taking your pill.
• Breakthrough bleeding or spotting that is new for you — worth mentioning at your next contact with your prescriber.
• Rash, itching or hives. About 10% of people report an allergic reaction to penicillin antibiotics, though only about 1% have a true allergy (GoodRx) — call your doctor immediately rather than waiting.
• Severe diarrhea with watery or bloody stools, with or without fever and stomach cramps, which can appear up to two months or more after treatment ends — call your doctor immediately or get emergency medical treatment (MedlinePlus, National Library of Medicine).
• Difficulty breathing or swallowing, wheezing, or swelling of the lips, face, tongue or throat — seek emergency care now (Medical News Today).
• Feeling faint, light-headed, losing consciousness or having a seizure, which can accompany anaphylaxis along with low blood pressure (Children's Minnesota) — seek emergency care now. A serious reaction can start within minutes or within 24 hours.

1. Keep taking both as prescribed for now. Do not stop or skip either the antibiotic or your contraception without talking to your doctor or pharmacist first — an unfinished antibiotic course and a missed pill each carry their own problems.
2. Call your pharmacist. This is usually the fastest route and needs no appointment. Tell them the exact antibiotic name, your contraceptive method and brand, and whether you have vomited or had diarrhea since starting. They can confirm whether your specific antibiotic is a rifamycin.
3. Contact your prescriber if anything has gone wrong with absorption, if you have had unprotected sex you are worried about, or if you use a progestin-only method. Ask: does this antibiotic affect my method? Do I need backup, and for how long? What should I do if I vomit after a dose?
4. Consider backup while you wait for an answer. Using condoms during the antibiotic course and for seven days afterwards is a conservative approach some clinicians suggest (Persly) — it is not required by the main guidance, but it costs nothing to use one.
5. Write down the dates of any vomiting, severe diarrhea or missed doses. Bring that list to your appointment; it is more useful than trying to recall it.