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Fertility drug 'may double cancer risk'
Fertility drugs used by thousands of women to boost their chances of having a baby could double the risk of cancer.
The 23,000 women who take clomiphene every year have twice the chance of developing thyroid cancer, a study suggests.
The drug is the most commonly used fertility treatment in Britain.
It is often given to 'kick-start' ovaries before before women start more invasive procedures such as IVF.
The study also warns that the hormonal treatment progesterone, used to improve a woman's chances of having a baby under IVF, may raise the risk of thyroid cancer by up to ten times. However, more research is needed to confirm this.
The study of more than 54,000 women, published in the medical journal Human Reproduction, last night led to calls for better regulation of fertility drugs.
Researchers from Copenhagen University Hospital found 29 cases of thyroid cancer among women aged 28 to 55, with most diagnoses taking place seven years after fertility treatment.
The majority had taken clomiphene, also known by the brand name Clomid, before developing the cancer, which kills 200 British women a year.
In two of the 29 cases the women had taken progesterone, which is used to make the woman's body have a period in preparation for implantation of an IVF embryo.
The study looked into thyroid cancer because the gland plays a crucial role in hormonal balance, which may affect fertility.
Clomid is commonly used as a first treatment for women who are "subfertile" - those who have tried for a baby for more than a year without success.