How the injection works
When you get the contraceptive injection, synthetic progestogen, called medroxyprogesterone acetate, is injected either directly into the muscle (Depo Provera) or under the skin (Sayana Press).
This hormone is slowly absorbed into your bloodstream. While it can have an effect on many of your body’s organs, the effects that make it an excellent contraceptive are:
it stops you from producing an egg (ovulating) so there’s no egg to fertilise
it thickens the cervical mucus, stopping sperm from travelling from the vagina to the womb
it thins the lining of the womb – a fertilised egg can’t implant in a womb with a thinned lining (this is a back-up measure as most people won’t produce an egg at all)
How the injection affects your body
The hormone travels all over the body in the bloodstream, with both positive and negative effects. It:
stops the pituitary gland in the brain from starting ovulation, so no egg is released
thickens cervical mucus so sperm can’t get through
thins the lining of the womb so an egg can’t implant
reduces the risk of cancer of the lining of the womb
reduces the risk of cancer of the ovary
What happens to the progestogen hormone once it's in your body?
The hormone is absorbed into the bloodstream slowly over 13 weeks from the site of the injection.
Your blood takes the hormones all over your body, changing your hormone levels and leading to other health benefits, risks and side effects.
Your liver changes the hormones so they’re ready to be expelled from the body.
Hormones are lost from your body through your bowel, so you have to keep having the injection every 13 weeks to keep hormones at the levels that prevent pregnancy.
How the injection affects your ovaries and uterus
During ovulation the cervical mucus is slippery and the lining of the womb (uterus) gets thicker so it’s easier for an egg to implant.
The hormones in the injection cause changes in both the womb and ovaries. No egg is released, the mucus is thicker and the lining of the womb is thinner.
There are 3 types of injectable contraceptive
There are 2 brands of contraceptive injection commonly issued in the UK, and both deliver the same dose of medroxyprogesterone acetate. The 3rd brand is not commonly used in the UK.
Sayana Press – inject it yourself
Sayana Press is a subcutaneous injection, which you inject yourself under the skin of your abdomen or thigh every 13 weeks.
Depo Provera – injected by a healthcare professional
Depo Provera is an intra-muscular injection, which is injected by a healthcare professional into your buttock every 13 weeks.
Noristerat – injected by a healthcare professional
This contains a different hormone and is given every 8 weeks. It’s rarely used in the UK. Our information does not include this type of injection.
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