Only barrier contraception, such as condoms, prevent STIs from being passed on.
Some things make the injection less effective - that means it doesn't work properly!
A small flexible rod containing the hormone progestogen. It is inserted under the skin, usually on the upper part of the arm, and will last for up to three years.
The hormone progestogen stops ovulation (release of an egg) and thickens the mucus around the cervix, which makes it difficult for sperm to get into the womb.
The patch is 99% effective when used correctly.