Perhaps, they reason, the treatment -- say a vitamin, or an aspirin -- could have a physiological effect not yet found in studies.

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"At this stage I would be circumspect about its utility, even if there is a proven physiological or pharmacological effect, " he said.

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During the 17-plus-months of their virtual voyage, the crew took part in various studies to assess the effect their isolation was having on their psychological and physiological well being.

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