How many days after antibiotics does birth control work

Medically Reviewed By William C. Lloyd III, MD, FACS

— Written By Megan Freedman

Updated on August 4, 2020

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Do you know that you can get pregnant while taking antibiotics, even if you’re taking birth control pills? Not all antibiotics have this effect. But to be safe, before starting an antibiotic, tell your healthcare provider that you are on the pill and ask if you should use a backup form of birth control temporarily.

How do antibiotics affect birth control pills?

The birth control pill, commonly called “the pill," is a medication women take once a day to prevent pregnancy. One way birth control pills prevent pregnancy is by preventing ovulation—when your body releases an egg that can be fertilized by a male sperm. Some antibiotics can interfere with your menstrual cycle, causing you to ovulate even though you’re on the pill.

Which antibiotics are more likely to interfere with the pill?

Not all antibiotics will increase your chances of getting pregnant on the pill. However, one particular antibiotic, called rifampin (Rifadin, Rimactane), can interfere with your menstrual cycle and reduce the effectiveness of birth control pills. While rifampin is less commonly prescribed today, other types of medications may also have the same effect. These include the antifungal griseofulvin, antiretrovirals, and the herbal remedy St. John's Wort.

You may wonder how an antibiotic could possibly affect your birth control pill. In the case of rifampin, scientific studies have shown that it decreases blood levels of both ethinyl estradiol and the progestins contained in birth control pills. If their blood levels fall too low, the hormones can’t do their job of preventing ovulation. The mechanism is complex, but basically rifampin stimulates your liver to process and get rid of these hormones faster than normal.

The data are not so clear for other antibiotics, so the interaction remains controversial. Unlike rifampin, there aren’t conclusive scientific studies for other antibiotics. But there are various theories about how other antibiotics might interfere with your birth control pill. Some of these theories are even more complex than rifampin’s interaction. Basically, the theories suggest that antibiotics may decrease the amount of hormones your body absorbs. However, none of them are solidly proven.

Talk with your healthcare provider or pharmacist before starting an antibiotic. Ask if it will change the effectiveness of your birth control pills or other hormone-based birth control you may be taking, such as an implant, shot or patch. If you have any worries or doubts about taking an antibiotic with your birth control pill, be safe and use a backup method of birth control. It can’t hurt and it may put your mind at ease.

How can I prevent pregnancy if I'm taking antibiotics?

It is not likely that you will get pregnant while taking antibiotics on the pill. But it doesn't hurt to be extra cautious. To be safe, use a backup method of birth control while you take your antibiotics and for up to a week after you finish. Backup birth control options you can try include condoms, sponges and spermicide. You can buy these forms of birth control without a prescription at many grocery stores and drugstores.

Do not stop taking your birth control pills even if you are temporarily using a backup method of birth control. Take your pills exactly as you always have to avoid messing up your cycle.

When it comes to pervasive old wives’ tales (or totally believable rumors) about birth control, you’ve definitely heard at least once that you can’t get pregnant while on your period and the pull-out method is a way to prevent pregnancy. Thankfully those myths have been debunked time and time again, but there’s one that still sends women panicking to their ob-gyns (or to Dr. Google): Antibiotics can seriously mess with your birth control.

For Real, Do Antibiotics Affect Birth Control?

Since antibiotics and birth control pills are both metabolized in the liver, there used to be a concern that the two would interact with each other—and become less effective—if taken at the same time, says Maria Sophocles, M.D., N.C.M.P, FACOG, board-certified ob-gyn in Princeton, New Jersey.

Luckily, that isn’t usually the case. Most broad-spectrum antibiotics (antibiotics that can be used to treat a variety of infectious diseases, like those you'd take for ear, throat, sinus, or urinary tract infections) do not alter the effectiveness of oral contraceptives, the patch, or the ring, according to the Centers for Disease Control and Prevention (CDC). On the flip side, birth control has not been found to make an antibiotic less effective, either, says Dr. Sophocles. “It’s been studied a lot, and the good news is that there’s really only one antibiotic, called rifampin, that substantially alters the effectiveness of a birth control pill,” she says.

Rifampin is an antibiotic most commonly used to treat tuberculosis or occasionally in people who have meningitis infections in their noses or throats. It works by increasing certain enzymes in your body, according to the United Kingdom National Health Service (NHS). These enzymes increase the breakdown of estrogen in the liver—and because estrogen is one of the active hormones in the birth control pill that helps prevent ovulation, it makes the contraceptive less effective, says Sherry A. Ross, M.D., ob-gyn and author of She-ology.

Rifampin doesn't just affect combined oral contraceptives (birth control pills that contain both estrogen and progestin); rifampin can also make progestin-only pills (sometimes called the "minipill"), implant, patch, and vaginal ring less effective at preventing pregnancy, according to the U.K. NHS. FWIW, although it's not entirely clear why, if you’re using an IUD or progestin-only injection, you’re still protected, according to the CDC.

Though rifampin is concern number one when it comes to antibiotics and birth control, another antibiotic called rifabutin (which helps prevent or slow the spread of Mycobacterium avium complex disease, an infection in patients with HIV, and may be used in combination with other medications to treat tuberculosis) could also reduce the effectiveness of birth control. Still, research on the antibiotic is limited, and existing studies show a smaller impact on effectiveness than rifampin, according to the CDC. At the end of the day, though, “whether you’re on the IUD, the pill, the implant, whatever it may be, antibiotics, in general, are fine,” says Dr. Sophocles.

Though the guidelines are clear, if you’re still nervous about the two impacting one another, consider taking your birth control pill in the morning and the antibiotic in the evening, rather than at the same time, says Dr. Sophocles. Or, of course, you can ask your doctor if you should change the way you take your contraception when you're being prescribed the antibiotic.

(P.S. did you know that supplements can mess with your prescription medications, including your birth control?)

How Long Do Certain Antibiotics Affect Birth Control?

Exactly how long an antibiotic can affect birth control depends on the amount of time you’re taking it, says Dr. Ross. And since your doctor may recommend you take rifampin for several months (seriously, most people with TB are prescribed the drug for at least six months because the bacteria die so slowly), using a second method of protection, such as a condom, is super important. “If I were on rifampin and I were taking any type of birth control, I would be using back-up contraception, and I would be very careful,” says Dr. Sophocles.

To prevent unwanted pregnancy, Dr. Sophocles recommends using backup contraception the entire time you’re on rifampin or rifabutin and the first few days you’re off the antibiotic, while Dr. Ross suggests sticking to it for up to a month afterward. After all, missing a single pill can put you at risk of pregnancy, and it can take at least seven days after you first start taking birth control pills for them to effectively protect against pregnancy.

And while you don’t normally need to use additional contraceptives if you’re taking antibiotics other than those two, there’s no harm in playing it safe, especially since condoms prevent STIs, too.

“If you have an easy opportunity to use back-up contraception, use it,” says Dr. Sophocles. “It’s never going to hurt to use it, and it’s a lot better than having an ‘oops.’”

How long after taking antibiotics will birth control work?

How long are you protected after taking antibiotics?