Finished cephalexin on the 1st day of my 4th week on my current birth control cycle, i start a new pack next week, when will bc be effective again?

Antibiotic Quest: Rifampin is the only antibiotic proven to affect OCPs (oral contraceptives). Rifampin may also decrease effectiveness of transdermal and vaginal ring preparations. In spite of anecdotal reports of OCP failure, other antibiotics have not been proven to affect the pharmacokinetics of OCPs. Therefore, your OCPs should be effective currently.