Received my first shot of depo birth control and was prescribed antibiotic at the same time. had my last period 2 weeks ago. how long should i use a backup method?

Pregnant ?'s: While some antibiotics may interfere with oral bcp's potential effectiveness, injection of progestins (depo-progesterone shot) avoid this problem, but the "depo shot" will act by prevention of implantation by the action of progestins on the uterine lining. If symptoms of pregnancy occur, a serum HCG test will confirm pregnancy with certainty.