What is misoprostol? Key side effects and common uses

Induction of labour and termination of pregnancy

One of the many actions of prostaglandins is the contraction of smooth muscle. Smooth muscles are found in blood vessels and airways and are not normally controlled voluntarily like skeletal muscles are. Smooth muscle is also found in the uterus (womb) which means misoprostol can cause contractions of the uterus. This would be dangerous for the foetus during a pregnancy, but this action is taken advantage of at the end of pregnancy to induce, or kick-start, labour. Misoprostol pessaries (vaginal tablets) are widely used in hospital to induce labour in pregnant women who have gone over their due date.

Termination of pregnancy

It follows that if misoprostol can kick-start labour at the end of pregnancy, the muscle contractions it stimulates could cause the loss of a foetus early in pregnancy. What's more, misoprostol is known to cause birth defects in some pregnancies that do continue to full term.

For these reasons, misoprostol should not be taken by pregnant women and women of child-bearing age must use effective contraception if taking misoprostol.

However, this effect could make misoprostol a practical and less invasive way of performing medical abortion. Indeed, although it is not licensed by the manufacturer (off-label) to do so, it is widely used for this purpose under medical supervision.

It is usually used in combination with another medicine called mifepristone. This is a type of steroid that 'softens' the muscles in the uterus and cervix and prepares them for contractions. Studies have shown that using a regimen of mifepristone followed by misoprostol is 95-98% effective in causing abortion.[i] Misoprostol is less effective when used alone, typically around 88% effective if used early enough. This leaves around 12% of women requiring more invasive abortion methods or continuing to full-term with a risk of birth defects.

As well as the usual side effects of misoprostol, this type of 'medication abortion' will cause heavy bleeding and tummy cramps, similar to a natural miscarriage. The main risk, which occurs in around 1% of cases, is severe bleeding that requires medical attention. There is also a risk of ectopic pregnancy and around a 1% chance of birth defects in a pregnancy that continues to term.

Read on to find out more about misoprostol and termination of pregnancy (abortion).