The progestin in the shot causes you not to ovulate, and long term use has been known to be hard to bounce back from. Definitely talk with your doctor about the “Pros & Cons”.
It did for me. It also screwed me up for two years. It tricks your body into thinking you are already pregnant. No one should be on it more than a couple years.
If you don't want to have kids any time soon it's okay.
I got pregnant on the shot and miscarried. I also bled for months on it and lost a ton of weight. It didn't work well for me.