Adni asked
Will My Antibiotic Affect My Birth Control? I heard certain antibiotics like rifamycin can cause big problems.
Answer
You are correct when you say that the most significant interactions with oral contraceptives are with the rifamycin family of antibiotics. Fortunately, they aren't commonly used and are typically indicated for infections such as tuberculosis and meningitis.
They cause problems with oral contraceptives because they are well known to be what is called an "metabolizing enzyme inducer". They increase the ability of certain metabolizing enzymes to break down different drugs. Both estrogens and progestins are partially broken down and metabolized in the liver and when taking alongside an enzyme inducer, they are metabolized and eliminated from the body more quickly. This can cause significant effects such as breakthrough bleeding, spotting and a loss of contraceptive efficacy.
The medication you have listed in your question, amoxicillin, is in the penicillin family of antibiotics, and is not an enzyme inducer. It is thought however, that certain broad spectrum antibiotics such as amoxicillin can affect how estrogens and progestins are metabolized because they change the normal flora of bacteria in your gut, therefore altering how oral contraceptives are metabolized. The final result is thought to be a decrease in concentration of the hormones in oral contraceptives possibly leading to spotting and a loss of contraceptive efficacy.
There has always been some debate on this matter and there was always little evidence regarding the validity of the possible interaction. The consensus appears to be that most antibiotics DO NOT alter the levels of hormones from oral contraceptives in your body.
In terms of your statement regarding stomach problems being an indicator of poor oral contraceptive absorption, we have not heard this before, and it is not stated in any of the scientific literature we are aware of. Nausea/vomiting is a common side effect of antibiotics and general and really couldn't be used to determine whether or not oral contraceptives are being absorbed.
There have been two major studies regarding the possible interaction: This one and this one.
The studies concluded that most antibiotics (such as amoxicillin) do not change levels of oral contraceptives and that back-up contraception is not necessary if contraceptives are used reliably during antibiotic use. It is interesting to note however, the second study did find a SMALL number of patients in which penicillin type drugs did alter the levels of hormones in the body (although did not cause any clinical effects). They recommend in their study that given the serious consequences of unwanted pregnancy, short term back up contraceptive use may be justified.
The majority of data certainly points to most antibiotics not causing any problems in people. There does appear to be some evidence in a SMALL sample of people that antibiotics may cause some changes in hormone levels however. Although most likely not necessary, it's is a personal decision if you wish to use backup birth control. Current guidelines do state that in most cases backup birth control is not necessary.
