What is the real-world probability of my getting pregnant while taking antibiotics and oral contraceptives?
I take Seasonale as my primary form of birth control, so I only have a period every three months. I was recently put on Bactrim (sulfamethoxazole and trimethoprim) to treat an ear infection. My doctor warned me, when writing out the prescription, to use a backup barrier method of birth control "until my period would normally arrive" (ie, for another three weeks).
I've pubmedded this to death, and I can't find *any* studies indicating a link between antibiotics and a decrease in OC efficacy (with the exception of rifampin); I did find several finding no (statistical) drop in efficacy, and some attribute that decrease not to the antibiotics but to women forgetting to take the pill when they feel crappy.
I know, I know, the easy answer is, "Just use a condom for three weeks." But I really want to know! Is this a myth? If so, how did it get started--typical CYA? Does anyone have any experience with (not) getting pregnant while mixing BC and antibiotics?
Other possibly influential factors:
- I'm slender (5'9", 130 lbs), so in general not worried about the dosage of contraceptives being too low for my body weight.
- I just started a new pack of contraceptives the day before starting the antibiotics.
- I am absolutely religious about taking the pill at the same time, *every* day.
So what are the odds?