Don’t use it for more than 2 years total.
FDA warning indicated that a woman should use Depo-Provera for more than two years only if other contraceptive methods are inadequate for her. This warning also applies to new lower-dose formulations of DMPA.
Black Box Warnings
In the Epocrates app, the Black Box warning reads as follows.
BMD Loss: “Significant BMD loss possible; greater loss with increased duration of use. May not be completely reversible. Unknown if use during adolescent / early adulthood will decrease peak bone mass and increase future osteoporotic fracture risk; Only use as long-term birth control method (>2 years) if other methods are inadequate.”
FDA Warning
http://www.arhp.org/Publications-and-Resources/Clinical-Fact-Sheets/DMPA
http://www.arhp.org/Publications-and-Resources/Clinical-Fact-Sheets/DMPA