Stem-cell tourism involving patients who travel to developing countries for treatment with unproven and potentially risky therapies should be more tightly regulated, international health experts said on Wednesday.

With hundreds of medical centers around the world claiming to be able to repair damaged tissue in conditions such as multiple sclerosis and Parkinson's disease, tackling unscrupulous advertising of such procedures is crucial.

These therapies are advertised directly to patients with the promise of a cure, but there is often little or no evidence to show they will help, or that they will not cause harm, the 15 experts wrote in the journal Science Translational Medicine.

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Some types of stem cell transplant - mainly using blood and skin stem cells - have been approved by regulators after full clinical trials found they could treat certain types of cancer and grow skin grafts for burns patients.

But many other potential therapies are only in the earliest stages of development and have not been approved by international regulators.

"Stem cell therapies hold a lot of promise, but we need rigorous clinical trials and regulatory processes to determine whether a proposed treatment is safe, effective and better than existing treatments," said one of the 15, Sarah Chan of Britain's University of Edinburgh.

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The experts called for global action, led by the World Health Organization, to introduce controls on advertising and agree international standards for the manufacture and testing of cell and tissue-based therapies.

"The globalization of health markets and the specific tensions surrounding stem cell research and its applications

have made this a difficult challenge," they wrote. "However, the stakes are too high not to take a united stance."