Yes, it depends. First of all, it depends on the birth control method you are using. In the case of oral contraceptives (ocp's) some antibiotics (azithromycin, ciprofloxacin) can increase ocp metabolism and therefore reduce their levels in the blood, increasing chances of ocp failure. You should always ask your doctor or pharmacist about all the medications you are taking before taking anything new.
Is it true that birth control can become less effective when taking antibiotics? And if so how much less effective?
Double your pleasure. When you try to double your pill taking pleasure by taking the oral contraceptive pill and antibiotic pill at the same time, the antibiotic will be preferentially absorbed and the oral contraceptive not. Thus, the BC pill is less effective and pregnancies can and do occur. Read more...