How Long Do Antibiotics Affect Birth Control Pills?

Summary: The interaction between birth control pills and antibiotics is controversial. However, most doctors and pharmacists will recommend, at the very least, that you use a barrier form of birth control for as long as you are on the antibiotic.

Dear Curtis: I take generic Ortho Tri Cyclen. Last week I came down with a sinus infection and the doctor gave me an antibiotic called Amoxicillin. The warning label on my bottle tells me that antibiotics can decrease the effectiveness of my birth control. But for how long?

Believe it or not, a lot of your question hasn’t been nailed down for sure. Even after all these years.

Here’s what I mean. When you take an antibiotic it’s job is to kill bacteria, right? Well, they sometimes have the tendency to kill both good and bad bacteria. Some of those good bacteria reside in your gut and we think that by killing those good bacteria you can end up affecting the blood levels of birth control pills.

So while this theory sounds good there is only one problem, it hasn’t been definitively proven in large scale studies just yet.

However, because it’s up in the air, I would still recommend that you use a back-up form of barrier contraception (condoms for example) for as long as you are taking the antibiotic, in addition to another seven days beyond when you stop taking the antibiotic.

Until Better Evidence Exists

Some doctors or pharmacists might disagree with that, which is fine. But here is what I base my opinion on:

  1. Some reports show that failure rates on oral contraceptives alone are around 2.3%. Oral contraceptives with antibiotics bumped it up to 2.6%. This 0.3% increase is not significant – unless your in that 0.3%.However, one study, which went backwards and looked at some of the available studies which had been performed, showed that using contraceptives and antibiotics together had a failure rate of up 1.6%.

    Frankly, bluntly, the evidence we have to date shows that the numbers can fluctuate a bit. But it also clearly shows that in some cases, there is a smaller risk of seeing higher failure rates. In the medical community in general, when that happens almost every practitioner will ‘cover their bases’, let you know about it and warn you that it can happen. That’s also the tact I take until we have evidence that suggests otherwise. (1,2)

  2. The antibiotic that you are taking, Amoxicillin, is one of only a handful of antibiotics that has been implicated in some of these interactions. So, in your particular case I think it would be very wise to use a back-up method.

One other word of warning: if you ever happen to switch from your current oral contraceptive to one of the newer, much lower-dosed oral contraceptives I think this recommendation will become even more important.

In short, using a back-up method of birth control while you are taking antibiotics – and for a week after you finish – is a smart step to take just for peace of mind until definitive proof comes out that it is not necessary. I think this is even more true for women who have had pregnancies while on birth control in the past or who suffer from breakthrough bleeding while taking birth control pills.


(1) Weisberg E., Interactions Between Oral Contraceptives and Antifungals/Antibacterials: Is Contraceptive Failure the Result? Clinical Pharmacokinetics, Volume 36, Number 5, May 1999 , pp. 309-313(5)
(2) Helms SE, Bredle DL, Zajic J, Jarjoura D, Brodell RT, Krishnarao I. Oral contraceptive failure rates and oral antibiotics. J Am Acad Dermatol 1997; 36:705-10.


  1. […] rest assured, that this is – unfortunately – a common problem with birth control pills (the other one being drug interactions like antibiotics). And, also unfortunately, one of the biggest reasons why failure (pregnancy) rates are higher than […]

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