The French-made pill, with the chemical name of mifepristone, was 100 per cent successful in a study, involving more than 600 women, on the reliability and side- effects of three post-coital contraceptives.
The women had all requested such contraceptives within three days of having unprotected sex. One group was given RU486, and two other groups each received alternative treatments, involving the drug danazol, and a combination of oestrogen and progesterone. In each group, 11 pregnancies were expected.
In fact, there were none among those taking RU486, nine in the danazol group and five in the combined hormone group.
Side effects such as nausea, vomiting and breast tenderness were less frequent among the women who took RU486, although these women tended to report more disruption of subsequent periods than those in other groups.
The study, funded by the World Health Organisation and carried out by researchers at hospitals in Manchester, confirms results obtained by the University of Glasgow in similar research published last week.
The drug brings about abortion early in pregnancy by causing the embryo to become separated from the wall of the uterus. It is then expelled.
Kenneth Calman, the Government's chief medical officer, said in a report last month that one in five abortions carried out in the United Kingdom last year were on women aged under 20. Of these, almost one in 10 were girls under the age of 16.
In today's study in the British Medical Journal, the authors conclude: 'Knowledge and availability of post-coital contraception can reduce the numbers of unplanned pregnancies, and give health professionals the opportunity to encourage and supply ongoing effective contraception.'
RU486 is currently licensed in the UK only as an abortion agent and is given about eight weeks after conception.
Roussel, the manufacturers of the drug, would have to apply to the Medicines Control Agency for a change in the terms of its licence before the treatment could be routinely prescribed.Reuse content