A toxicologist from Leeds University has played down claims from Labour's shadow environment secretary Mary Creagh that the drug phenylbutazone causes cancer in humans.
Professor Alistair Hay said there is "no convincing evidence of carcinogenicity".
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Ukip leader Paul Nuttall also said he would want to get rid of Sharia courts.
Over 40,000 people picked up their packs to take part in the annual marathon in the capital.
The country's voters are concerned about security, the economy, and the 10% unemployment rate.