Only barrier contraception, such as condoms, prevent STIs from being passed on.
Any sexual contact without a condom may pass on STIs.
Some things make the injection less effective - that means it doesn't work properly!
Small plastic and copper device, sometimes T-shaped, which is inserted into the womb.
Also available is the intrauterine contraceptive implant, which consists of copper beads threaded on to a length of nylon, which is attached to the top of the womb.
The IUD stops sperm meeting an egg, or may stop an egg implanting in the womb.
The patch is 98-99% effective when used correctly.