Depo provera (generic name: medroxyprogesterone is a progesterone-like contraceptive). It increases the blood levels of progesterone, thus inhibiting the production of gonadotropin releasing hormone from the hypothalamus. The lower gonadotropin releasing hormone lowers the production of follicle-stimulating hormone and luteinizing hormone from the anterior part of the pituitary gland. This, in the end, prevents the ovulation and without an egg pregnancy is impossible.


The above explained is exactly what happens during pregnancy when progesterone levels are high and the normal menstrual cycle is inhibited.
Morning after pills might contain only progestin or both estradiol and progestin. Their aim is to delay ovulation or completely inhibit it, and to make the implantation impossible by affecting the inner lining of the uterus. Taking a morning after pill when taking Depo provera might result in unexpected bleeding, spotting, or other forms of unexplainable and irregular vaginal bleeding. Other than this, it might additionally affect the hormonal levels in the blood, so that intervention by a doctor might be needed to return them to the previous levels controlled by Depo provera.


The chances that you can get pregnant 3 days before the 12 week mark after continuous 3 years use are no bigger than the total risk of failure for Depo provera to prevent unwanted pregnancy, which is from 0.3 to 3 percent.


You might want to consult with your gynecologist about whether it is safe for your health to take the morning after pill when regularly taking Depo provera. S/he is the one who knows your reproductive health best and would be best in determining the benefits and risks from taking both Depo provera and the morning after pill.