Depo Provera is a long-acting hormone injection, the only method of birth control by injection currently available. The medication lasts three months, and it prevents pregnancy by hindering ovulation and making the lining of the uterus too thin for the egg to implant itself. Depo Provera is particularly useful for women who have difficulty remembering to take pills on a daily basis. And it's more effective than barrier methods such as the diaphragm or condom. The main side effect reported with Depo Provera is bleeding.
Many women experience unpredictable, periodic flow during the first three to nine months of use.
This occurs because the uterine lining becomes too thin in some women, and blood breaks through. Almost all women stop bleeding by the ninth month, provided that they receive regular injections every three months. Other common side effects are depression, weight gain, and bloating. Unfortunately Depo Provera can't be reversed once it's injected, so women who experience these symptoms can only wait until the injection wears off in three months. If a woman doesn't experience side effects from these, she probably won't from Depo Provera either.