Thanks for your question!
We have written about this before (the article can be found here) and the answer is no, Benadryl (diphenhydramine) will not decrease the effectiveness of your birth control pills (i.e. oral hormonal contraceptives).
You specifically asked about 'combination' birth control pills, meaning they contain both an estrogen and a progestin. Regardless of whether or not the birth control pill in question contains both or just a progestin, the answer is the same, there is no interaction.
In fact, there are no known interactions at all between Benadryl and any birth control pill product. You can feel safe taking them together.
Interactions To Know
While you have nothing to worry about when it comes to combining Benadryl with your birth control pills, there are several drugs that could potentially alter how effective they are.
We have written quite a few articles discussing these types of interactions, such as this one discussing the use of ciprofloxacin.
The overall consensus is that most antibiotics won't significantly alter the effectiveness of birth control pills, but there may be a small number of individuals more susceptible than others.
One study, published in Obstetrics & Gynecology, gives a very pointed recommendation regarding this:
"Pharmacokinetic studies of other antibiotics have not shown any systematic interaction between antibiotics and OC [oral contraceptive] steroids. However, 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."
Since the consequences of an unplanned pregnancy can be great, it is always a good idea to err on the side of caution (e.g. use backup birth control) when there is even the smallest risk of your birth control pills not working as intended.
As a final note on this antibiotic point, some less common ones are well-known to significantly decrease birth control hormone concentrations, such as rifampin.
These types of antibiotics aren't common, but if you are prescribed them and you are on birth control, your pharmacist and/or other health care provider will discuss what to expect and appropriate contraceptive methods.
Since Benadryl is available over the counter (OTC), it would be prudent to discuss if there are any OTC drugs that you need to be concerned about in regard to their potential to have an effect on your birth control pills. I list some potential interactions below:
- Acetaminophen: This is the active ingredient in Tylenol. Acetaminophen can increase estrogen concentrations, which may increase estrogen-related side effects, like nausea. This will not decrease effectiveness, however.
- Caffeine: Caffeine concentrations may be increased when combined with estrogen (which is contained in most birth control pills). This can increase caffeine-related side effects.
- Vitamin C: High doses of vitamin C (ascorbic acid) can increase estrogen levels, potentially increasing the risk of estrogen-related side effects.
- Cimetidine: This is the active ingredient in Tagamet. Cimetidine can increase estrogen levels and levels of cimetidine may be increased as well when used together.
- St. John's Wort: St. John's Wort is one of the most extensively used herbal products for a variety of indications, including depression. Several studies suggest that St. John's wort could decrease estrogen levels in the body and generally, the combination of it and birth control pills should be avoided.
So, overall, there aren't really any significant interactions between birth control pills and OTC drugs (with the possible exception of some herbal products, like St. John's Wort).
Some may increase estrogen-related side effects but when it comes to decreasing how well you are protected from pregnancy, there is little concern.
There is no known drug interaction between Benadryl (diphenhydramine) and oral contraceptive pills.
- Antibiotic and oral contraceptive drug interactions: Is there a need for concern? PubMed
- Drug interactions between oral contraceptives and antibiotics. Science Direct
- Interaction of St. John's Wort with oral contraceptives: effects on the pharmacokinetics of norethindrone and ethinyl estradiol, ovarian activity and breakthrough bleeding. PubMed
- Diphenhydramine Overview. PubChem
- Pharmacokinetic drug interactions with oral contraceptives. PubMed