How effective is it? Over 99% effective when used correctly. Less than 1 woman in every 100 will get pregnant each year using this method.
How does it work? It contains oestrogen and progesterone, which stops ovulation, thickens mucus in the cervix to prevent sperm reaching an egg and thins the lining of the uterus to prevent an egg implanting.
Advantages: Usually makes periods regular, lighter and less painful, reduces risk of cancer of the ovary, uterus and colon, fertility will return to normal when you stop taking the pill.
Disadvantages: Not suitable for very overweight women or smokers over 35 years, small risk of blood clots, breast and cervical cancer, can be side effects: headaches, nausea, mood changes and breast tenderness.
The pill can be less effective if it is missed, if you are vomiting or have diarrhoea. Some medicines make it less effective. Breakthrough bleeds are common in the first few months.