Hi, Depo-Provera is a hormonal injection which has contraceptive action to avoid pregnancy. It contains Medroxy progesterone acetate which is analogue of the hormone Progestin synthesized in the body which helps to sustain the pregnancy after fertilization of the egg. Menstrual cycle consists of Proliferative and Secretory phase. Menstruation is the beginning of the menstrual cycle during which the endometrial lining developed during the last cycle to accommodate the fertilized egg gets shredded along with its blood supply. In the same time, there is development of egg during this phase of the cycle. After attains the maturity, the matured egg is released for fertilization which is called as Ovulation. Secretory phase is the phase after ovulation during which the fertilization of the egg occurs and the zygote get implanted into the endometrium. Progesterone is the hormone of the Secretory phase which helps in endometrial development, cervical mucous changes and ovulation. Depo Provera injection has its contraceptive effect by suppressing the ovulation and release of eggs thereby inhibiting the fertilization, also inhibits the endometrial development and make the cervical mucous thick which in turn prevents sperm motility. Single shot of injection has its effect for 12 weeks. For continuous contraceptive effect, it should be repeated after every 12 weeks. It may take up to one year that the body adapts to the mechanism without any side effects. Side effects of the drug are nausea, vomiting, spotting, irregular periods, breast tenderness, etc. These symptoms will subside once the body gets adapted to the medicine. Visit the gynecologist, if the symptoms any of the side effect gets worse. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Pradeepraj, General & Family Physician