How it works
The injection releases a progestogen hormone (most commonly medroxyprogesterone acetate) slowly into the bloodstream. It mainly stops the ovaries from releasing an egg, and it also thickens cervical mucus and thins the lining of the uterus.
People who want effective contraception without a daily pill or a fitted device and who are not planning a pregnancy within the next year.
How effective is it?
Typical use
96%
In real life about 4 in 100 people get pregnant in a year with typical use.
Perfect use
Over 99%
About 0.2 in 100 when used exactly as directed every time.
See how it ranks against every method in the effectiveness chart.
How to use it
Get it from a doctor, nurse, or sexual health or family planning clinic; one type can be self-injected at home after training.
The injection goes into a muscle (usually the buttock or upper arm) or, for the self-injected type, just under the skin of the tummy or thigh.
Depending on when in your cycle you start, it may work straight away or you may need condoms or another method for the first 7 days.
Book your next injection before the current one runs out; how late you can be depends on the type, so ask your clinic and use another method if you are overdue.
To stop, simply do not have the next injection; its effects cannot be removed once given, so side effects may last until it wears off.
Pros and cons
Advantages
- Only needs to be given a few times a year, with nothing to remember day to day.
- Contains no estrogen, so it suits many people who cannot use combined methods.
- Not affected by vomiting, diarrhea, or most other medicines.
- Periods often become lighter or stop over time.
- Private: there is nothing to see or carry.
Disadvantages
- Does not protect against sexually transmitted infections (STIs); use condoms as well for that.
- Fertility can take up to about a year to return after the last injection, so it may not suit people planning a pregnancy soon.
- Long-term use is linked to a reduction in bone density, which mostly recovers after stopping.
- Side effects cannot be stopped quickly because the injection keeps working until it wears off.
- Weight gain is more commonly reported than with other methods.
Effect on periods: Bleeding often becomes irregular at first, and periods stop for many people the longer they use it.
Possible side effects
- Irregular bleeding or spotting, especially in the first months
- Periods stopping
- Weight gain
- Headaches
- Mood changes
- Soreness or a small lump at the injection site
Who it may not suit
A clinician checks your medical history before you start. This list is not complete.
May not be suitable if
- You want to get pregnant within the next year or so.
- You have, or are at higher risk of, osteoporosis; clinicians may be more cautious in teenagers and people over 45.
- You have, or have had, breast cancer.
- You have had a stroke, heart disease, or several risk factors for heart disease, or a past blood clot.
- You have unexplained vaginal bleeding that has not been checked, or serious liver disease.
Get medical help if
- You are late for your next injection; you may need to use another method or take a pregnancy test first.
- You have very heavy or long-lasting bleeding.
- You notice low mood or mood changes that worry you.
- You think you might be pregnant.
Frequently asked questions
How often do you need the contraceptive injection?
The most common type is given every 13 weeks in the UK, or every 12 to 13 weeks (about every 3 months) in the US. Another type, used less often, lasts 8 weeks. Your clinic will tell you the interval for the type you have.
How long does it take to get pregnant after the shot?
It can take up to about a year for fertility to return to normal after the last injection, although some people conceive sooner. It does not cause permanent infertility.
Does the contraceptive injection cause weight gain?
Weight gain is a recognized side effect of the most common injection and is reported more often than with other methods. It does not happen to everyone.
Does the injection affect bone density?
Using it for a long time can lower bone density. This mostly recovers after stopping, and clinicians usually review whether it is still the best option every couple of years.
Can I give myself the contraceptive injection?
One under-the-skin version can be self-injected at home after a clinician shows you how. Availability varies by country and clinic.
Sources
Reviewed 1 November 2026. We link to the guidance itself so you can check it.