abnormal uterine bleeding
"I've been on Depo for almost 12 years now. Bled / spotted non-stop for the first 6 months and a little after that. The side effects normalized after a full 12 months. Initially I gained weight by only on the first few months. When it's close to being time for my next injection I get PMS symptoms and every now and then,  I get "unusual" (unusual for me) side effects with an injection but it usually is only for 3 months and goes back to normal with the next one. I do,  however,  experience spotting / bleeding after sex sometimes. I've heard it can be from cervical capillaries that are made more sensitive from the drug but I don't know that there's firm research on it."