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In 1996, investigators from the Fred Hutchinson Cancer Research Center in Seattle studied 18,000 people who, because they had been exposed to asbestos, were—like those who smoked cigarettes—also at greater risk of lung cancer. Participants were given large doses of vitamin A, beta-carotene, both, or neither. The study ended abruptly when the safety monitors realized that those taking megavitamins had a dramatically higher rate of lung cancer (28 percent greater than those not receiving vitamins) as well as heart disease (17 percent greater).
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Paul A. Offit (Pandora's Lab: Seven Stories of Science Gone Wrong)