Providing contraception is a big part of my women’s health care practice. If you start your first pack the same day your period starts, protection starts immediately. That relates back to @nikipedia‘s comments about suppressing ovulation – at that point in your cycle, the egg has not started maturing. If you’re starting later in your cycle, give it a minimum of 2 weeks, ideally until the end of the first pack.
@westy81585, the belief that antibiotics reduce the effectiveness of OCPs is actually pretty much a myth, although even some physicians and pharmacists still warn women about it. The studies this belief is based on measured hormone levels in the bloodstream and break through bleeding as surrogate markers for pregnancy risk – they didn’t actually show increased ovulation or pregnancy. The studies were also quite small. There is no good evidence that taking antibiotics increases pregnancy risk. A very small percentage of women will have decreased blood levels of estrogen while on antibiotics due to effects on bacteria in the colon, and that might cause some breakthrough bleeding. But it turns out that progesterone is the most important hormone when it comes to pregnancy prevention – thickening the cervical mucus is just as important as supressing ovulation, if not more so.
Women who are on some anti-seizure drugs (not all of them) do have reduced OCP effectiveness because of more rapid metabolism of both estrogen and progesterone in the liver. St John’s wort and a few other meds can cause the same problem.
@Marina, Yaz is an OCP with a progesterone that has some unique effects, which can be both good and bad. It’s especially effective against acne and fluid retention and PMS, but in high risk women, it can cause increased levels of potassium in the bloodstream. High risk conditions would be certain hypertensive meds, and a few other meds. OCPs are safe and effective for most women.
Sorry for the long post – hope this isn’t TMI!