Information Sheet

Contraceptive injection

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The contraceptive injection is a hormone injection given every 12 weeks to prevent pregnancy.

How does it work?

The contraceptive injection contains a hormone like progestogen (one of the hormones made in the ovaries). The hormone is slowly released over 12 weeks and stops the ovaries from releasing an egg each month. It also thickens the mucus/fluid at the cervix (opening to the uterus/womb) so that sperm cannot get through to meet an egg.

How effective is it?

The contraceptive injection is over 99% effective if used correctly – that is, if 100 people used it for a year, only one of them might become pregnant. If it is forgotten or given late it may fail to prevent pregnancy, so in real life it may only be 96% effective. It is more reliable than the contraceptive pill (also known as “the Pill”) as you only need to remember to have the injection every 12 weeks, instead of taking a daily pill. No contraceptive is 100% effective.

Will it work immediately?

If the injection is given within the first five days of the menstrual cycle (where day one is the first day of your period), or within the first 5 days after an abortion or miscarriage, then it is effective immediately. If it is given at another time during your menstrual cycle, another form of contraception (such as condoms) should be used for the next 7 days to avoid unplanned pregnancy. Your doctor will discuss with you the best time to have the contraceptive injection, as they need to check you are not already pregnant.

How long will it last?

Each injection will provide contraception for a full 12 weeks. It is important that you have regular injections every 12 weeks to prevent unplanned pregnancy. You should make regular appointments with your doctor to ensure you get you next injection on time.

What are the side effects?

The most common side effect of the contraceptive injection is irregular bleeding, especially following the first injection. After one or more injections many people have lighter periods and in many cases your periods may stop altogether. This is not harmful. A small number of people can have longer periods or continual light bleeding for some weeks. In rare cases heavy bleeding may occur.

A small amount of weight gain may occur with the contraceptive injection, but a large increase in weight is uncommon. Other side effects are uncommon but some people headaches, acne, moodiness, a loss of libido, bloating and fluid retention have been reported.

If you are at all concerned about any symptoms you think may be caused by the contraceptive injection, you should contact us on 1300 003 707 or your doctor.

What if I want to stop using the contraceptive injection? Is it reversible?

Once the contraceptive hormone has been injected it cannot be reversed for 12 weeks and, due to a continuing low level of hormone in the body following the final injection, there may be a delay in the return of your periods .

When you stop using the injection it may take many months for your periods to return to normal (an average of 8 months) and up to a year for your fertility to return to normal.
Despite this, it is important to start using another form of contraception within 14 weeks of the last injection if you do not want to get pregnant. There is no way of knowing how soon your fertility will return.

Who should not use the contraceptive injection?

There are some circumstances and medical conditions where you should not use the contraceptive injection, before starting you should inform your doctor if you:

• May be pregnant
• Have or have had breast cancer
• Have had a history of any unusual vaginal bleeding
• Have had heart disease or a stroke
• Have high blood pressure
• Have severe diabetes
• Are planning on becoming pregnant in the near future
• Have had an allergic reaction to the contraceptive injection in the past.

Your doctor will talk with you about your contraceptive options and help you decide whether the contraceptive injection is the best method for you.

What if I am breastfeeding?

The contraceptive injection is safe in breastfeeding from birth.

What else should I consider?

Long term use may reduce the mineral content of bone in some people that is likely to be reversible when the injections are stopped. Because of this, if you are under 18 or over 45, the contraceptive injection may not be the best first choice of contraception for you.

Contraceptive injections offer no protection against STIs.

How is the contraceptive injection given?

Following the initial consultation and your consent, the doctor or nurse will administer a small, quick injection of the contraception into your buttock or into the muscle of your upper arm.

Where can I get it?

The contraceptive injection must be prescribed by a doctor from one of our centres, your local doctor or a family planning centre.

This page last edited: March 2023
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