(CNN) The US Food and Drug Administration is taking new action against dietary supplements, sending warning letters to companies who claim, without proof, that their products can prevent or treat Alzheimer's, diabetes and cancer, the agency announced Monday.

The FDA vowed to update its policies on dietary supplements, promising "one of the most significant modernizations of dietary supplement regulation and oversight in more than 25 years," according to a statement by FDA Commissioner Dr. Scott Gottlieb.

The FDA does not review dietary supplements before they hit the market, but it can intervene when products are deemed unsafe or carry false, misleading or unproven claims about their health benefits. The agency said these claims can cause real harm when people forgo approved treatments that have been proven safe and effective.

Roughly three-quarters of American consumers regularly take dietary supplements -- including 4 out of 5 older adults, Gottlieb said. The industry has swelled with tens of thousands of products collectively worth more than $40 billion, he added.

On Monday, the FDA sent 12 warning letters and five online advisory letters to companies marketing their products illegally as Alzheimer's treatments. In recent months, the agency has also gone after a number of other products, including male enhancement supplements and those claiming to treat opioid addiction.

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