Williams, who is suffering from severe stomach disorder and exhaustion, is defying doctors' orders to rest and instead will perform a duet with Australian disco diva Kylie Minogue, his spokesman says.

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Zelnorm, formerly known as Zelmac, treats irritable bowel disorder (IBS)--a combination of constipation, stomach pain and other severe symptoms that affects up to 15% of the population, mostly women.

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