riri asked
I am on birth control pills. I might have to start an antibiotic today that reduces efficacy. I will use a second barrier method for sex when i am on the antibiotic.However I had sex yesterday that relied only on the pills as protection. Should I be worried about reduced efficacy for sex I've had before taking the antibiotics? I know that sperm lives in the body 5 days, during which I will be on the antibiotic. I am also on my last active pill today, so I will start my placebo pills tomorrow.
Answer
There is no evidence that points to an individual needing to use back-up birth control (e.g. condoms, spermicide) before antibiotic use to prevent pregnancy. In fact, there is little evidence that antibiotic use lowers the efficacy of birth control pills at all (with the exception to the rifampin family of antibiotics).
One of the larger studies on the matter concluded the following:
"Available scientific and pharmacokinetic data do not support the hypothesis that antibiotics (with the exception of rifampin) lower the contraceptive efficacy of oral contraceptives."
The general consensus is that the number of pregnancies that occur with antibiotics and birth control pills is no greater than the number of pregnancies during the use of birth control pills alone, without concomitant antibiotics.
Nevertheless, one large review study in 2001 noted that data indicates that a small number of patients may experience unexplained decreases in hormone levels while taking antibiotics. From the study:
"Individual patients do show large decreases in the plasma concentrations of ethinyl estradiol when they take certain other antibiotics, notably tetracycline and penicillin derivatives. Because it is not possible to identify these women in advance, a cautious approach is advised."
Based on the available information, it doesn't appear that most antibiotics affect hormone levels from birth control. However, as unplanned pregnancy is a serious matter, and studies have suggested that there may be certain individuals susceptible to antibiotic induced lowering of hormones from birth control pills, back-up contraception while taking antibiotics may be a good idea to stay safe, just in case.
As a side note, the American College of Obstetrics and Gynecology recommends the use of back-up, non-hormonal contraceptives (e.g. condoms) only with the concomitant use of rifampin or griseofulvin (an anti-fungal).
Back-Up Contraception BEFORE Antibiotic Use?
The theoretical mechanism behind how antibiotics reduce birth control pill efficacy is based on how antibiotics reduce or alter our normal bacterial flora in the gut.
A reduction in gut bacteria is theorized to diminish how estrogen is metabolized, which then decreases how much is absorbed via enterohepatic circulation, resulting in lower blood concentrations of estrogen. This mechanism has not been confirmed however.
What is important about the theorized mechanism is that antibiotics would not affect the hormones from birth control pills that are already absorbed and in the blood. Therefore, there would be no need to use back-up birth control before or in preparation of antibiotic use as it would be unnecessary.
