Advantages:
It’s a very reliable method of birth control.
You only have to think about it once every 12 weeks.
It reduces menstrual bleeding.
Nobody has to know, so it’s a good method if your parents are against contraceptives, but you’ll have sex anyhow.

Disadvantages:
If there are serious side effects or if you are allergic you cannot stop right away!!! This is the biggest problem. You have to live with it till it’s worked out. That’s why many girls choose some other method of birth control.
It may lead to irregular periods and spotting. You may stop having periods after a while. For some this is felt as a disadvantage, for others as an advantage. But you can also get heavier and longer periods.
  Less common side effects: weight gain, hair loss, headaches, depressions, nervousness, dizziness, increased facial/body hair, increased or decreased sex drive, acne, joint pain, bloating, sore breasts, fatigue and other problems. Some women are allergic to Depo-Provera.
When you stop the injections it may take a while before you can become pregnant. It may even take up to a year. It does not lead to infertility, as some fear.
It doesn’t protect you against STD’s.