How effective any contraceptive is depends on how old you are, how often you have sex and whether you follow the instructions.
If 100 sexually active women don’t use any contraception, 80 to 90 will become pregnant in a year.
Perfect use: If the injection is always used according to instructions it is over 99 per cent effective. This means that less than four women in 1,000 will get pregnant over two years.
Typical use: If the injection is not always used according to instructions, about six in 100 women will get pregnant in a year.
The injection is a method of long-acting reversible contraception (LARC). All LARC is very effective because while it is being used you do not have to remember to take or use contraception.