The drug does cause side-effects in some patients, including, rarely, cancer of the lining of the womb, but the scientists said that the increased chance of preventing breast cancer returning far outweighed the risk.

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Emergency contraception is effective for the first 72 hours after sex, but is more likely to work the sooner it is taken, and can cause side-effects such as nausea and diarrhoea in some women.

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However, some of these can have unpleasant side-effects which cause people to stop taking their medication.

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Some existing medicines, such as the transplant drug cyclosporine, cause so many side effects that doctors are reluctant to use them.

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Kentucky lawyers Shirley Cunningham, William Gallion and Melbourne Mills, Jr. recruited some 440 clients after reports that fen-phen, a diet-drug combination, could cause life-threatening side effects.

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