I know that most antibiotics will interfere with birth control pills working properly. Anyone know why it does?
Also what other types of medication require a backup method in addition to the pill?
Most antibiotics do NOT interfere with oral contraceptives. With the exception of rifampin (a drug used to treat TB and similar infections), antibiotics generally have NO significant effect on birth control bill effectiveness.
Rifampin DOES lower the effectiveness of the BCP by virtue of its ability to stimulate the enzymes that break down the active ingredients of the BCP. Phrased differently, rifampin “revs up” the metabolism of the BCP thereby rendering it less likely to work.
Rifampin does this to lots of drugs, not just the BCP (look one third of the way down here for example)
In terms of other drugs’ effects on the BCP, look here.
I think the above should be changed to “Most antibiotics do NOT interfere with oral contraceptives in MOST people.”
Whereas there is no reliable scientific evidence that most antibiotics interfere with birth control pills in the population in general, my personal experience would indicate that that’s not always the case.
I’ve used antibiotics twice while using the birth control pill. The first time (I don’t remember which antibiotic it was) I got pregnant. Okay, maybe I was just unlucky and it had nothing to do with the antibiotics–it’s possible. The second time I was taking Macrobid on the third week of my cycle, and I got my period a week early, while still taking the active birth control pills. Not just spotting, a full-blown period. Maybe the birth control pill was still effective, but frankly I think that if I’m getting my period when I’m not supposed to, that’s an indication that something’s going on. By the way, the birth control pill I was taking was Alesse, a low-dose pill.
I think the scientific reasoning for why antibiotics MIGHT interfere with the birth control pill has to do with the way the liver metalobizes the drug or something.
Hmmm… My doctor has always told me to use a backup form of birth control while on antibiotics.
In addition, a good friend of mine managed to get herself pregnant while on antibiotics. Her doctor hadn’t told her (or she hadn’t remembered) about the antibiotic/pill connection. She has a 10 year old now!
I’ve been warned that antibiotics can affect it, and since a lot of antibiotics give me diarrhea, and diarrhea can cut down on the effectiveness, I’m going to stay on the side of paranoia. I’m paranoid enough that I don’t have sex without a condom and pill just so that I really really really don’t have to worry about getting pregnant. I don’t need to worry about the other benefits of condoms at the moment, but not getting pregnant is a biggie.
THere’s a warning label on all my birth control patches that antibiotics may interfere with BCP working.
Warning labels, tradition, and anecdotes aside, there is precious little evidence to support the idea that antibiotics interfere with the action of the BCP (with the notable exception of rifampin, as mentioned). There are reports of penicillin and tetracycline causing BCP failure, but the rates of this occurrence are what would be expected anyways as a “background” failure rate.
Here are some reviews which summarize the (lack of) evidence:
-
From the Journal of the American Academy of Dermatology 2002
-
From The Dental Clinics of North America 2002 (note that dentists often prescribe antibiotics. This paper gives them some medicolegal basis re: lack of interference with contraception).
In my opinion, much of the motivation for advice along the lines of “use a back up form of contraception when taking antibiotics” is based on fear of medicolegal consequences (and habit). It’s easier to dispense such advice than it is to prove that that a BCP failure was not due to antibiotic interference. (Remember, there will always be a “background failure rate” for any BPC). To be honest, though, if I was in the same situation, I’d probably do the same. Still, it would be to protect me more than to protect the patient.
FWIW, I have a standing prescription for Macrobid, and checked and double checked to see if it would interfere with my BCP. I didn’t find one source of information that said it would interfere, nor did my doctor or pharmacist. (I have had other antibiotics interfere with my period - Cipro specifically)
Yeah, but doesn’t diarrhea alone cut down on the likelihood of pregnancy? I now I don’t tend to hop into bed with people when I have the runs. 
Karl Gauss, I know you’re a lot more qualified than me, and I don’t want to start any kind of fight, but hear me out.
I know Rifampicin induces CYP450, but I was under the impression (from pharmacology lectures) that other antibiotics affected the effectiveness of the COCP by killing off the intestinal bacteria necessary for the conversion of the ethinyl estradiol to a more biologically active form.
The journal of Obstetrics and Gynaecology abstract you cited did say that in some women the serum levels of ethinyl estradiol were significantly lowered by concurrent use of penicillens and tetracyclines and that as it was impossible to determine who these women were in advance “caution is advised”.
That is NOT the same as saying that contraceptive failure after using these antibiotics can be completely explained away by “background” failure.
Macrolides, as others have mentioned cause nausea, vomiting and GI upset as frequent side-effects. Even if the drugs themselves do not interfere with the pharmacokinetics of the drug, these side effects may impede absorption. It is reasonable to suggest that use of a Macrolide, if it caused vomiting or diarrhoea, could lead to contraceptive failure.