This might "translate into one or two more years of life" for a regular supplement taker, says lead author Philippe Autier of the International Agency for Research on Cancer.

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In Europe, about half the adult population is currently overweight, and urban areas of many developing countries have a similar prevalence, according to the International Agency for Research on Cancer.

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And there is no animal evidence either that it is a carcinogen, he added, saying that "phenylbutazone is not classifiable as to its carcinogenicity" according to the International Agency for Research on Cancer.

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Dr Paolo Boffetta, of the Unit of Environmental Cancer Epidemiology of the International Agency for Research on Cancer in Lyon, France, who worked on the study, said: "We do not really know if asthma as such causes the increased risk".

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The World Health Organization's International Agency for Research on Cancer is coordinating Interphone.

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