I have taken my pills at the same time each day and my only method for birth control has been the pill. For example, if I was sexually active yesterday and I started the antibiotic today, is there a risk for pregnancy? Or is the risk only present after I begin to take the antibiotic? More so, my worry is the the antibiotic will some how affect my birth control for the previous times that I was sexually active. I also started the placebo week today.