I use the injectable AND a condom, EVERYTIME! and I don't even menstruate...Not because I'm worried my bf is fooling around, I've known him since we were kids and trust him, but its always better to be safe than sorry. I learned that lesson the hard way... Fell pregnant at 19 through the pill. Yes I took it diligently, 8pm every night, no I wasn't on any antibiotics. Apparantly the pill doesn't work in my family and my older sisters failed to mention this "minor" detail to me, until AFTER I announced I was pregnant

So my advice is obvious, use 2 forms UNLESS you don't mind an unexpected baby or a STI or 2...

Btw, HUGE condom market, shop around, I am very finicky about condom types as I'm sensitive, but personally prefer the "skin-feel" ones and studded variations from Durex.