Augusta Healthcare for Women
The birth control shot often is called by its brand name Depo-Provera®. With this method you get injections, or shots, of the hormone progestin in the buttocks or arm every 3 months. A new type is injected under the skin. The birth control shot stops the ovaries from releasing an egg in most women. It also causes changes in the cervix that keep the sperm from joining with the egg.
The shot should not be used more than 2 years in a row because it can cause a temporary loss of bone density. The loss increases the longer this method is used. The bone does start to grow after this method is stopped. But it may increase the risk of fracture and osteoporosis if used for a long time.
Reference: Dept. of Health and Human Services