Depo Question

I jsut recently got my first injection. I am 18 and am taking the shot for birth control purposes, since I have app. 10 years of school left. I have heard alot about the shot becoming defective when u take antibiotics or do other things. Can anyone tell me things that cause the shot not to work? I`d like ot watch out for these things. My doctor told me that there was nothing that made the shot stop working, but i know of 2 people that have gotten pregnant on it in the past 3 months. Someone gimme some info plz... Thanks