Medroxyprogesterone acetate (Depo-Provera) for contraception is administered as microcrystals, suspended in an aqueous solution, which dissolve very slittlely. The dose of medroxyprogesterone acetate for contraceptive purposes is 150 mg intramuscularly every 3 months. The injection should be given within the first 5 days of the current menstrual cycle, otherwise a back-up method is necessary for 2 weeks. The effectual contraceptive level is maintained for 4 months, providing a safety margin for reliable contraception. The efficacy of this method is equal to that of sterilization. The mechanism of action is the same as with all progestin-only methods, except the circulating level of the progestin is high enough to effectually block the LH surge, and therefore it is unlikely that any patient will ovulate with this method. Depo-Provera also affects the endometrium and cervical mucus, producing barriers to implantation and sperm penetration, as with Norplant. Suppression of FSH is not as intense as with the combination oral contraceptive, therefore follicular growth is maintained sufficiently to produce estrogen levels comparable to those in the early folllicular phase of a normal menstrual cycle. Symptoms of estrogen deficiency, such as vaginal atrophy or a decrease in breast size, do not occur.