If used correctly the contraceptive injection is more than 99% effective. This means that less than one woman in 100 who use the injection will become pregnant in a year.
The injection lasts for eight weeks or 12 weeks (depending on the type), so you don't have to think about contraception every day or every time you have sex.
It can be useful for women who might forget to take the contraceptive pill every day.
It can be useful for women who can't use contraception that contains oestrogen.
It's not affected by medication.
The contraceptive injection may provide some protection against cancer of the womb and pelvic inflammatory disease.
Side effects can include weight gain, headaches, mood swings, breast tenderness and irregular bleeding. The injection can't be removed from your body, so if you have side effects they'll last as long as the injection and for some time afterwards.
Your periods may become more irregular or longer, or stop altogether (amenorrhoea). Treatment is available if your bleeding is heavy or longer than normal – talk to your doctor or nurse about this.
It can take up to one year for your fertility to return to normal after the injection wears off, so it may not be suitable if you want to have a baby in the near future.
Using Depo-Provera affects your natural oestrogen levels, which can cause thinning of the bones.
The injection does not protect against sexually transmitted infections (STIs). By using condoms as well as the injection, you'll help to protect yourself against STIs.
How the injection works
The contraceptive injection is usually given into a muscle in your bottom, although sometimes it may be given in a muscle in your upper arm. The contraceptive injection works in the same way as the implant. It steadily releases the hormone progestogen into your bloodstream. Progestogen is similar to the natural hormone progesterone, which is released by a woman's ovaries during her period.
The continuous release of progestogen:
stops a woman releasing an egg every month (ovulation)
thickens the mucus from the cervix (neck of the womb), making it difficult for sperm to pass through to the womb and reach an unfertilised egg
makes the lining of the womb thinner so that it is unable to support a fertilised egg
The injection can be given at any time during your menstrual cycle, as long as you and your doctor are reasonably sure you are not pregnant.
When it starts to work
If you have the injection during the first five days of your cycle, you will be immediately protected against becoming pregnant.
If you have the injection on any other day of your cycle, you will not be protected against pregnancy for up to seven days. Use condoms or another method of contraception during this time.
After giving birth
You can have the contraceptive injection at any time after you have given birth, if you are not breastfeeding. If you are breastfeeding, the injection will usually be given after six weeks, although it may be given earlier if necessary.
If you start injections on or before day 21 after giving birth, you will be immediately protected against becoming pregnant.
If you start injections after day 21, you will need to use additional contraception for the following seven days.
Heavy and irregular bleeding is more likely to occur if you have the contraceptive injection during the first few weeks after giving birth.
It is safe to use contraceptive injections while you are breastfeeding.
After a miscarriage or abortion
You can have the injection immediately after a miscarriage or abortion, and you will be protected against pregnancy straight away. If you have the injection more than five days after a miscarriage or abortion, you'll need to use additional contraception for seven days.
Advantages and disadvantages of the injection
The main advantages of the contraceptive injection are:
each injection lasts for either eight or 12 weeks
the injection does not interrupt sex
the injection is an option if you cannot use oestrogen-based contraception, such as the combined pill, contraceptive patch or vaginal ring
you do not have to remember to take a pill every day
the injection is safe to use while you are breastfeeding
the injection is not affected by other medicines
the injection may reduce heavy, painful periods and help with premenstrual symptoms for some women
the injection offers some protection from pelvic inflammatory disease (the mucus from the cervix may stop bacteria entering the womb) and may also give some protection against cancer of the womb
Using the contraceptive injection may have some disadvantages, which you should consider carefully before deciding on the right method of contraception for you. These are as follows.
Disrupted periods
Your periods may change significantly during the first year of using the injection. They will usually become irregular and may be very heavy, or shorter and lighter, or stop altogether. This may settle down after the first year, but may continue as long as the injected progestogen remains in your body.
It can take a while for your periods and natural fertility to return after you stop using the injection. It takes around eight to 12 weeks for injected progestogen to leave the body, but you may have to wait longer for your periods to return to normal if you are trying to get pregnant.
Until you are ovulating regularly each month, it can be difficult to work out when you are at your most fertile. In some cases, it can take three months to a year for your periods to return to normal.
Weight gain
You may put on weight when you use contraceptive injections, although some women lose weight. Use of the injections may be associated with an increase in weight of up to 2–3kg over one year.
Depo-Provera, oestrogen and bone risk
Using Depo-Provera affects your natural oestrogen levels, which can cause thinning of the bones but it does not increase your risk of bone fracture (breaking a bone). This isn't a problem for most women because the bone replaces itself when you stop the injection, and it doesn't appear to cause any long-term problems.
Thinning of the bones may be a problem for women who already have an increased risk of developing osteoporosis (for example because they have low oestrogen, or a family history of osteoporosis). It may also be a concern for women under 19 because the body is still making bone at this age. Women under 19 may use Depo-Provera, but only after careful evaluation by a doctor.
Other side effects that some women report are:
headaches
acne
tender breasts
changes in mood
loss of sex drive
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