The contraceptive injection, or Depo-Provera, contains three months worth of hormones—a synthetic form of progestin, called depo-medroxyprogesterone acetate (DMPA). To get an injection, you need to see your doctor every 11 to 13 weeks. If you miss an injection make an appointment with your doctor as soon as possible. The main side effect is an increased risk of pregnancy; if you miss a dose for multiple weeks or even months, you may re-experience the same side effects you had when you started this contraceptive injection.
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If you miss an injection of Depo-Provera, you don't have increased amount of hormones that suppress ovulation. While ovulation usually resumes three to six months after stopping the contraceptive injection, it is possible to get pregnant if you miss a shot, according to the American Pregnancy Association. Planned Parenthood states that your doctor may give you a pregnancy test if it has been two weeks or more since your last injection, or emergency contraception if you had unprotected sex in the past 120 hours. To reduce your risk of an unwanted pregnancy, use a secondary form of birth control, like condoms, until your next injection.
One of the side effects of the contraceptive injection is an irregular period, which can range from no period to heavier periods, according to the APA. The drop in hormones from missing your injection may result in some changes; if, for example, you normally do not get your period while on Depo-Provera, you may have some breakthrough bleeding or increased light spotting.
Other Side Effects
Planned Parenthood notes that side effects from starting the contraceptive injection wear off after 12 to 14 weeks once your body adjusts to the higher levels of hormones. If you miss your next shot for a long period, such as multiple weeks, and the hormones from the previous injection cycle out of your body, your side effects may return. These include breast tenderness, headaches, fatigue, nausea and dizziness. If you have any of these side effects, they should leave when your body readjusts.