Does cephalexin affect sprintec birth control pills?

No risk, no worry. It's an outdated urban myth that antibiotics interfere with effectiveness of birth control pills. They do not, except occasionally with rifampin, an infrequently used antibiotic for certain infections like tuberculosis. Lamisil (terbinafine) certainly has no effect. See https://www.drugs.com/article/antibiotics-and-birth-control.html.

Related Questions

I'm on sprintec birth control pills, am I still protected if I had unprotected sex while on the last row (the sugar pills)?

Yes. The birth control pill should prevent ovulation if taken properly. Theoretically, you should not have ovulated and should be getting a period soon. During the last week of the pill pack, you should not be at the fertile time of your cycle so should not be able to get pregnant. Read more...