how long does it take for the birth control shot to work
A Brigham and Women's Hospital answered. Depo-Provera is a hormone injection used for birth control. It is administered starting within the first five days of your period, then every 12 weeks thereafter. Working similar to a birth-control pill, Depo-Provera has the hormone progestin, which changes the lining of the uterus, making it difficult for sperm to fertilize an egg. In the weeks between shots, you do not have to use other contraceptive methods. It is 99% effective in preventing pregnancy.