How effective is it? Over 99% effective – less than 4 women per 1000 will get pregnant over two years.
How does it work? It releases progesterone (reproductive hormone), which thickens mucus in the cervix to stop sperm reaching the egg, and thins the lining of the womb to prevent implantation. It can also stop eggs being released (ovulation).
How long does it last? Depo-provera lasts for 12 weeks. Noristerat lasts for 8 weeks.
How does it affect periods? They may stop, be irregular or last longer than usual.
How does it affect fertility? When removed, periods and fertility will take time to return (up to 1 year).
How is it put in and taken out? The hormone must be injected into a large muscle – usually the buttocks. It cannot be taken out so any effects of the injection will last for 8 or 12 weeks.