How effective is it?
Over 99 per cent effective if taken according to instructions. Less than one woman in 100 will get pregnant in a year.
How does it work?
Contains the hormone progestogen, which thickens cervical mucus to prevent sperm reaching an egg and thins the lining of the uterus (womb) to prevent a fertilised egg implanting. In some cycles it stops ovulation.
What are the advantages?
No serious side-effects.
Can be used by women who cannot use estrogen.
Can be used by women who smoke and are over 35.
You can use it if you are breastfeeding.
Periods may stop, or be irregular, light or more frequent.
What are the disadvantages?
May be temporary side-effects such as acne, weight change and headaches.
A small increased risk of ectopic pregnancy if it fails.
May get ovarian cysts.
Anything else I should know?
It needs to be taken at the same time each day.
Not effective if taken over three hours late (12 hours for Cerazette) or after vomiting or severe, long-lasting diarrhoea. Some medicines may make it less effective.