As long as you keep getting the shot on time, you can always have sex. It is effective the whole time you are on it. If you are ever late getting your next shot, that is when you might want to use a backup method.

I was told by my Dr that i am fully protected from shot to shot as long as I get it on time. Although just as a precaution (which i don't know if it really matters)... is that my dr gives me a little card with about a 2 week time span colored in - in which i need to get my next one. I always go in as soon as I can to get my next one - I never wait until the last minute. Just a thought.

That's a very smart idea. Procrastinating with your birth control is a formula for disaster!