Meredith asked
Hello, I was prescribed the antibiotic amoxicillin for my wisdom extraction and I hear that most antibiotics do not diminish the effectiveness of birth control pills. The antibiotic family of rifamycins I have read are the only known antibiotics that have been shown to weaken effectiveness. I was reading that because there is little evidence antibiotics have an effect on birth control pills there have been new guidelines set; they now say that the only way it could diminish the effectiveness is if someone experiences stomach problems from the antibiotic could then have the possibility of the pill not being absorbed properly. Could you please tell me if amoxicillin can reduce the effectiveness of my birth control pills (Necon 1/35)? And if I should therefore use another form of contraception for the seven days after I have taken my last pill of amoxicillin ? Thank you so much!
Answer
You are correct when you say that the most significant interactions with oral contraceptives are with the rifamycin family of antibiotics.[1]
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 a "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.[2]
Penicillin Family Antibiotics And Birth Control Pills
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 the 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.[3]
Antibiotic-Induced GI Problems
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. However, most birth control pill products do give specific directions if you vomit within a certain time frame of taking a pill.
What Do The Studies Say?
There have been two major studies regarding the possible interaction:
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.
Final Words
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.
