Contraceptive injection
3-minute read
What is the contraceptive injection?
The contraceptive injection prevents pregnancy by injecting a synthetic version of the hormone progestogen, called Depot medroxyprogesterone acetate, or DMPA. The injection is also called Depo.
Depo prevents the body from producing its own hormones and releasing eggs from the ovaries. This is also how the contraceptive pill works.
The injection thickens the fluid at the entrance to the uterus (womb), which stops sperm from entering. It also thins the lining of the uterus, making it difficult for a fertilised egg to attach and develop.
It is given as an intramuscular injection into the buttock or the upper arm, and over the next 12 weeks the DMPA is slowly released into the bloodstream. To prevent pregnancy, an injection must be given every 12 weeks.
The injection is usually given during the first 5 days of the menstrual cycle, so it can start working straight away. If you have the injection at some other time in your cycle, it can take up to 7 days to start working.
Types of contraceptive injection
There are 2 birth control injections available in Australia:
How well does the contraceptive injection work?
The contraceptive injection is fairly effective if given every 3 months. If 100 women were to use it for 1 year, however, between 1 and 6 are still likely to become pregnant.
The contraceptive injection does not protect you against sexually-transmitted infections (STIs). You should use condoms for additional protection against STIs, although even condoms won't protect you from every type of infection.
Advantages and disadvantages of the contraceptive injection
The main advantages:
- It is highly effective.
- It can be used by women who can’t take oestrogen.
- You only need to have the injection once every 3 months.
- It is safe for use by most women, including those who are breastfeeding.
- It usually stops menstrual bleeding, so is useful for women who have heavy periods or period pain.
- Depo Provera may reduce the risk of ovarian cancer, endometrial (uterine) cancer, endometriosis and pelvic infection.
- It can be used if you’re taking some medications that make the pill or implant less effective.
The main disadvantages:
- It changes the pattern of the periods — they might become more frequent or longer lasting, then stop completely.
- If you stop using it, it can take a while to get pregnant – it can take 18 months for fertility to return.
- It can cause side effects such as weight gain, moodiness, decreased sex drive, headaches and acne.
- It might cause bone thinning if used for a long time.
- You have to visit a doctor every 3 months for an injection.
You shouldn’t have the injection if you are already pregnant, have a bleeding disorder, are taking anticoagulant medication, have had some types of cancer or other medical problems.
If you miss an injection, or stop having the injections
You need to have the injection every 12 weeks if you want to prevent pregnancy. It can be given up to 2 weeks late, but if you wait any longer than that, it's best to use condoms until the injection has had time to start working again.
When you stop getting the injection, it can take several months for your periods to return to normal, and even longer to become pregnant.
Resources and support
For more information, see Family Planning NSW.
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Last reviewed: January 2021
