I went through the EXACT same thing (almost) 5 months ago. I took the antibiotics had sex (more than once), realized about the antibiotic and looked up its interactions, freaked the hell out, got Plan B, and worried until my period came. It did. :)

I think that the interactions of BC and antibiotics are not well known because there aren't women lining up to be in studies to see if birth control will fail. So doctors are going on what women report having done. (I have a cousin who got pregnant while on the pill, but I would bet big money she wasn't taking it with any regularity, but she still swears up and down that "the pill doesn't work.")

I'm on an antibiotic again right now. I'll still use barrier protection when I have sex, but I know now not to freak out so much. The jury is still out on the interaction effects, but it's looking more like there really isn't too much of a connection.

First of all, I'm sorry that you were so freaked out and I'm glad that everything worked out okay. I'm also glad you have a supportive boyfriend.

I don't know why pharmasicts are telling all of us this if it's not true. About a year ago I had to go on antibiotics for a wound on my foot. This was the same week that I got back from a summer internship where I hadn't seen my boyfriend for 3 months. When I was picking up the antibiotics I asked my pharmasicts, who is normally very cool about answering any questions I have, if the antibiotics would screw up my birth control. He looked horribly uncomfortable, said something about well, you should use something else for that, and hurried away. I think I embarressed the man. I really can't be the only young women who has asked him this question, can I? So strange.

I really wish that there was more information out there about this.

I'm steaming right now that your PCP did not tell you this information when s/he prescribed it. Every time I have had antibiotics over the past 6 years (and 2 doctors), they have sternly told me to take it until I finished, eat some yogurt and remember it affects birth control until the end of my current pack.

It's been studied enough that we know it's a warning that needs to be made every time. You shouldn't have to ask the dumb, uncomfortable pharmacist or Google, this is your doctor's responsibility to make you aware.
It should also be clearly on the label of the bottle and the bag you get from the pharmacy.

Some antibiotics have a possible effect, most don't. The pharmacists and doctors tell you to be safe because they don't always know. They would rather err on the side of caution. The best thing would be to look up your specific antibiotic and see if it has interactions with bc at all. (or ask your doctor, but doctors don't know all of the possible interactions of every antibiotic with bc, which is why they usually err on the side of caution when prescribing antibiotics)

nyuszi, it's very possible the doctor told her and she just forgot. I mean, it's written right on the prescription labels anyway according to law (unless that's a state law here?). I forget stuff like that all the time, even when I have been told.

The thing is, antibiotics MIGHT lower the effectiveness of birth control, and some definitely do-- like the ones for TB. But they can't do studies, like ElleStar pointed out, so all they know is some might and some probably don't, and they don't know how much, so they figure it's safer and better for the woman if they say "use a barrier method when on antibiotics" than to not say it, or try to explain the problem, and risk the woman not understanding or not using backup and maybe getting pregnant as a result.

It's not like using a barrier method will kill you, and the worst that happens is you forget, take EC, and worry for a couple weeks. That's a lot better than being pregnant IMO.

I'm with nyuszi on this one. It's amazing to me how much misinformation doctor's give out about women's health. My sister got pregnant on the pill while on antibiotics, and the first time I went on the pill my doctor didn't tell me that antihistamines can lower the effectiveness of the pill. Since I've been breastfeeding for the past 11 months I've received the most ridiculous, innacurate, and contradictory advice. At the only women's clinic in this town the male doctors routinely prescribe regular brith control pills to nursing moms instead of the low-dose type that won't make your milk supply drop. One of them told me I could take cold or allergy medicine with no risk to my milk supply or the baby. Then when I talked to my midwife/lactation consultant 2 days later (she had been on vacation when I talked to that doctor) she was horrified and said that any medication that's intended to dry up your sinuses will also dry up your milk, and that many of them contain ibuprofen, which babies can't have. Great advice from doctors whose specialty area is supposed to be women's health.

Definately ask questions. A few years ago I had to take anitbiotics several times. Most pharmacists told me to use a backup method until the end of the pill cycle. However, one pharmacist told me that it did depend on the type of antibiotic, and that she herself was on the same one that I was prescribed, and that she'd been on it for years since it was prescribed for acne, and that she'd never gotten pregnant. So after that I relaxed a bit.

The problem is this: The risk of this interaction is very, very low for MOST women. But there are some women in which hormone levels can really drop when they are taking a course of certain antibiotics.

When they looked at estrogen levels in controlled trials most women taking short courses of certain antibiotics + the pill, had clinically insignificant lowering of estrogen levels. But a few women did experience significant drops, which could permit ovulation. The problem is there is no way to tell which women are susceptible to this effect.

Also, there have been many reports coming from post-marketing surveillance (reports of women getting pregnant while on the pill) to indicate that warning women of this interaction is warranted. But even this data is disputed because the numbers are so low that statically it's difficult to say if the drug interaction was the cause and not something else.

The best clinicians can do is give the information to you and let you decide. Wouldn't you want to know that there is a possibility of birth control failure even if it's small?

And keep in mind that many medical studies are conflicting as well. So that is why sometimes you ask 1 question and get several answers.

Medicine is not an exact science. Bottom line is that for most women this is not a problem. For a tiny few women it is a problem. There is simply no way to know which women to warn, so everyone gets the message instead.

I thought I'd post some supportive evidence.

Obstet Gynecol. 2001 Nov;98(5 Pt 1):853-60.
Drug interactions between oral contraceptives and antibiotics.

You can call it controversial data, but it's certainly not an old wives' tale.