Depo-Provera becomes effective immediately if administered within the first five to seven days of the start of a woman's menstrual period. If the injection is administered at any other time, using a backup method of birth control is necessary for at least a week.
Depo-Provera is a form of birth control that is administered as an injection every three months. Each injection contains a synthetic form of the hormone progestin. It works by blocking ovulation and thickening the cervical mucus to block sperm. It is 99 percent effective in preventing pregnancy when a woman receives injections regularly. This method of contraception does not provide protection against sexually transmitted diseases.