Hormones (either progestogen alone or a combination of progestogen and oestrogen) are injected into the muscle of the arm or buttocks. 

Progestogen-only injectable contraceptives (POIs) protect against pregnancy for three months at a time, whereas combined injectable contraceptives (CICs) require an injection each month. 

Contraceptive injections act in the same way as oral contraceptives to prevent pregnancy and have similar potential side effects. 

Although the benefits generally outweigh theoretical concerns, data suggests that POIs can decrease bone mineral density in women aged under 18, which could put these women at risk if POIs are continued over the long-term. 

If this is a concern, then CICs may be a better option. 

One of the main advantages of both injection types is that there is no visible sign that the person is using this method, but it does require a clinic visit every one or three months. 

The injection usually reduces blood loss and cramps during menses and menstruation may stop altogether for some women.  

Effects on pleasure are similar to oral contraceptives.