The injection I know of is depo-provera. It's progesterone only.
I was forced to come off this because it can affect your bone density if you're on it for a long time (5 years I was on it)
It also messed up my fertility and caused me to gain a lot of weight when my lifestyle hadn't changed at all. (around 50lbs gain, it was horrible, gone now, still no change in lifestyle)
When I came off it I bled for 2 months solid and thought I was going to lose my mind.
I can't recommend it really. It's AWESOME while you're on it, no periods, and so convenient to get. But there's madness going on in the background that's hard to fix.