The U.S. Food and Drug Administration announced a crackdown Monday on dietary supplements. Two out of three American adults take them, but public health officials warn some products contain unapproved ingredients and don't work as promised.

In a statement announcing the FDA's action, Commissioner Scott Gottlieb said 12 warning letters, and five advisory letters had been sent to supplement companies whose products "are being illegally marketed as unapproved new drugs" because they claim to "prevent, treat or cure Alzheimer's disease, as well as health conditions like diabetes and cancer."

The FDA found one company claimed its product had "proven effectiveness against numerous deadly viruses." Another touted "reducing symptoms of cognitive decline."

In the 25 years since the FDA began regulating dietary supplements, they have become a $40 billion dollar industry.

Gottlieb said in an online statement that he's concerned that "changes in the supplement market may have outpaced the evolution of our own policies and our capacity to manage emerging risks."

The trade group that represents supplement manufacturers applauded any effort by the FDA to take what it calls 'bad actors' out of the marketplace.

"When you have companies who don't abide by quality standards or make unreasonable claims for their products it hurts consumer confidence for all of the legitimate products," said Steve Mister of the Council for Responsible Nutrition.

While that industry group said it supports any crackdown on 'bad actors,' its representative said they won't support proposals for what it calls "unwarranted regulatory burdens for industry," like government approval being required before new products hit the market.