Depo-Provera is one of the most effective, reversible contraceptives. If administered correctly, the effectiveness rate is 99.997%. Depo-Provera does not offer any protection against STDs.
A hormone is injected that prevents the ovaries from releasing eggs. It also thickens the cervical mucus, which also keeps sperm from joining with an egg.
Less common side effects include weight gain, sore breasts, headache, depression, dizziness, skin rashes, vaginal dryness and an increase or decrease in sex drive.