Contains the hormone progestogen, which thickens cervical mucus to prevent sperm reaching an egg, and thins the lining of the uterus to prevent a fertilised egg implanting.
In some cycles it stops ovulation.
Advantages
Over 99 % effective if taken according to instructions.
Can be used by women who cannot use estrogen.
Can be used by women who smoke and are over 35 yrs.
You can use it if you are breastfeeding.
Disadvantages
Periods may stop, or be irregular, light, or more frequent.
May be temporary side effects such as spotty skin, breast tenderness, weight change and headaches.
May get ovarian cysts.
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.