What's the name of the drug she is using for her acne?
Most antibiotics probably do not affect birth control pills as stated on this Mayo Clinic site, the last paragraph. They do list one which is known to affect birth control, but is not widely used.
Then too, is the question of her using her birth control correctly, on time, which only she can answer. There is always a small risk of pregnancy. She can take a home pregnancy test if her period is more than a week late.
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