Contraceptive injection

The contraceptive injection contains only the hormone progestogen. The Injection is given by healthcare personnel in the upper arm or the buttocks every 12 weeks. The hormones are injected into the muscle and seep into the bloodstream over time to prevent ovulation. Contraceptive injections are not recommended for those under the age of 18.

Bleeding when using contraceptive injections
Most women who use contraceptive injections will eventually experience no bleeding at all. However, most will experience reduced or increased bleeding initially, especially after the first injection. The bleeding may be a nuisance, but it is not dangerous. 

Source: sex og samfunn