Most injections for contraception work by injecting a form of progestin, which is a synthetic (man-made) version of the natural hormone progesterone.
This article is part of our Contraception Series!
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Note: the information below was sourced from references (1-5), unless stated otherwise.
What is it?
Most injections for contraception work by injecting a form of progestin, which is a synthetic (man-made) version of the natural hormone progesterone. This is also called the progestin-only injectable; a widely-known brand is Depo Provera. The hormone is injected by a doctor (or other medical professional) in your muscle or in the fat layer under the skin (depending on the type), about every three months.
Some injectables are given with a different frequency or contain different hormones; a monthly injectable with both progestin and estrogen (combined injectable) is available in many different countries (you may know this as Cyclofem or Mesigyna). Which one you can get depends on the country you’re in and your personal circumstances.
The injection takes only a couple minutes, counting from the preparation of the injection to aftercare. Injections into muscle are typically a bit more painful than injections in the fat layer, but most people find the pain to be very manageable. No physical examinations are usually necessary.
Some doctor’s offices allow patients to inject themselves after appropriate training. If you’re interested in doing it yourself, ask your doctor about the possibilities.
How does it work?
For the progestin-only injectable, the injected hormone is typically a synthetic hormone called depot medroxyprogesterone acetate. This suppresses ovulation and thickens the mucus in the cervix. This prevents the release of an egg, and prevents sperm from coming up into the uterus and fallopian tubes to fertilize any egg that did get released. The hormone also makes the endometrium thinner, which prevents implantation of an egg that did get fertilized.
Combined injectables, with both progestin and estrogen, work similarly.
How effective is it?
With perfect use, injectables are very effective.
Progestin-only injectables (such as Depo Provera) are 99.8% effective in preventing pregnancy, meaning only 2 people* out of 1000 will become pregnant within a year. With typical use, the protection rate is lower: 96% (40 per 1000 people* per year will become pregnant).
For combined injectables (with two hormones, such as Cyclofem and Mesigyna), perfect use leads to less than 1 pregnancy per 1000 people* per year (99.9% effective) and about 30 pregnancies per 1000 people* per year for typical use (97% effective).
These differences in protection rates (perfect use versus typical use) are mainly because the efficacy of the injection decreases significantly if you’re not in time for your new injection. Your risk of pregnancy therefore largely depends on how consistent you are in getting injections. If that’s difficult for you, you could choose an implant or an intrauterine device, instead.
What are possible side-effects?
It is possible that you experience some irregular bleeding (you may stop bleeding entirely eventually) or gain weight. With long-term use, there may be some loss of bone density, which is reversible. You may experience headaches, tenderness in your breasts, abdominal discomfort, or mood changes.
The injections can also reduce heavy menstrual bleeding and cramping, and can be effective at reducing pain if you have endometriosis. Contraception injections also reduce the risk of endometrial and ovarian cancer.
Note that only a portion of people experience (some of) these side-effects, and some people will experience side-effects not listed here. Whether you will, depends on many different factors.
When can't I use it?
When you have unexplained bleeding from your genitals, have had diabetes for a long time (over 20 years), have or have had breast cancer, or have or have had thrombosis, heart or vessel disease (including high blood pressure), stroke, some rheumatic diseases, or liver disease.
Sometimes, you can get injections even if you have one of these things. Talk to your doctor to find out if your health issue would definitely mean you can’t get injections.
Does it work immediately?
This depends on when you got your injection (note: the following situations apply to the progestin-only injection, such as Depo Provera. Different situations might apply to the combined hormone injectables). If you got your first injection within seven days of starting your menstruation, you’re protected immediately. If you got your first injection after more than seven days since starting your menstruation, you need to use another method of contraception (for example, a condom) if you’re having sex in the seven days following your first injection. You also need to do this if you’re more than three weeks postpartum and aren’t breastfeeding.
For all injections after the first injection, they still work when given within two weeks of the date you were supposed to get your injection. If your injection is over two weeks late, you need to use another method of contraception (for example, a condom) if you’re having sex in the seven days following that injection. If it works better for your planning, you can get your injection earlier than you’re supposed to. This is safe. You only need to make sure that your next injection is within the next 3 months.
Check with your doctor which situation applies to you if you’re not sure.
What happens to my fertility if I stop using it?
When you stop getting your injections, you will be able to become pregnant again within a few months up to a year. The average time it takes until you’re able to conceive is about nine months (counted from the last injection). The reason it takes longer to regain fertility compared to other hormonal methods, is that the hormone that’s injected stays in the body for longer.
If you have obesity, becoming fertile again may take longer.
Does it protect against sexually transmitted diseases (STDs)?
No. Hormones alone do not protect against STDs. If you’re having sex with someone new or untested, use a barrier method as well.
*People, here, means anyone who is able to become pregnant, including girls, women, and non-binary people and transgender men who still have their uterus, vagina, and ovaries.
Are you curious about other methods to protect yourself from an unwanted pregnancy? Read about other birth control options here!
This article is pending medical review.
Written by Juliëtte Gossens
Reviewed by Sophie Oppelt and Selina Voßen
Edited by Juliëtte Gossens
McFarlane I (ed.). Seeing the unseen: The case for action in the neglected crisis of unintended pregnancy. United Nations Population Fund. 2022. Available from: https://www.unfpa.org/sites/default/files/pub-pdf/EN_SWP22%20report_0.pdf
Hacker NF, Gambone JC, Hobel CJ (eds.). Hacker & Moore’s Essentials of Obstetrics & Gynecology. 6th ed. Philadelphia: Elsevier; 2016.
Hoffman BL, Schorge JO, Halvorson LM, Hamid CA, Corton MM, Schaffer JI (eds.). William’s Gynecology. 4th ed. New York: McGraw-Hill Education; 2020.
Centers for Disease Control and Prevention. The United States Medical Eligibility Criteria for Contraceptive Use, 2016 (US MEC). Available from: https://www.cdc.gov/reproductivehealth/contraception/mmwr/mec/summary.html
Centers for Disease Control and Prevention. 2016 U.S. Selected Practice Recommendations for Contraceptive Use (U.S. SPR). Available from: https://www.cdc.gov/reproductivehealth/contraception/mmwr/spr/summary.html
Please note: the information we provide to you here is for educational purposes only. If you’re experiencing any discomfort or have any complaints or questions about your health, please contact your doctor or other relevant health professional. We don’t provide medical advice.