That is a very good question David.
There are some antibiotics that do interact with certain oral birth controls.
Cephelexin is in the group of broad-spectrum antibiotics called cephalosporins. Some broad-spectrum antibiotics may reduce the efficacy of combined (progesterone + estrogen) oral contraceptives by impairing bacterial flora responsible for recycling of estrogen in the large intestine. Therefore, the drug tends to be removed from the system more quickly and reduces the effectiveness of the birth control.
However, Plan B is in a category by itself since it is only taken for 2 doses and the main purpose is emergency contraception. Cephelexin has a low percentage of interacting with Plan B but as a precautionary measure you should be using a second form of contraception (i.e condoms, spermicidal agents) for 7 days. Hope this helps you both.
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