My boyfriend takes really strong acne medication. On the label it says it can cause birth defects. But is this only if you're the girl that's taking it. Or does it mean for the guy too? Will it mess up his sperm or something?

Its more probable if a pregnant female is taking it. Though, it is possible that it could alter sperm.
Its best to ask his doctor for specific information concerning the drug.

Yes, it is very possible that it can cause birth defects ESPECIALLY if the medication is Accutane.

Many medications do indeed alter the sperm and can cause birth defects. Most doctors will not prescribe accutane, for instance, without a form of birth control.