This medicine is commonly known as 'the morning after pill'. It is used to reduce the chances of becoming pregnant after unprotected sexual intercourse. It contains the active ingredient levonorgestrel, which is a synthetic derivative of the naturally occurring female sex hormone, progesterone.

It is thought to work by preventing ovulation and fertilisation and also by altering the lining of the womb, depending on which stage of the menstrual cycle the woman is at. In a woman's normal menstrual cycle, an egg matures and is released from the ovaries (ovulation). The ovary then produces progesterone, which prevents the release of further eggs. Levonorgestrel, tricks the body processes into thinking that ovulation has already occurred, as it provides high levels of synthetic progesterone. This prevents the release of eggs from the ovaries. Levonorgestrel also increases the thickness of the natural mucus at the neck of the womb (cervix), making it more difficult for sperm to cross from the vagina into the womb. By preventing sperm entering the womb, successful fertilisation of an egg, leading to pregnancy, is less likely. Levonorgestrel is also thought to alter the lining of the womb, preventing it from being prepared for a fertilised egg. This means that if an egg is released from the ovaries and is fertilised, it cannot implant into the womb and therefore pregnancy is avoided. The whole process from fertilisation to implantation in the womb can take up to three days, so the morning after pill can only stop a pregnancy for up to 72 hours after unprotected intercourse. The sooner it is taken after unprotected intercourse, the more effective it will be.

Formal Chemical Name (IUPAC)
(8R,9S,10R,13S,14S,17R)-13-ethyl-17-ethynyl-17-hydroxy-1,7,8,10,11,12,13,15,16,17-decahydro-2H-cyclopenta[a]phenanthren-3(6H,9H,14H)-one

References

http://www.netdoctor.co.uk/medicines/100004487.html

http://news.bbc.co.uk/1/hi/health/3253932.stm