The Food and Drug Administration said Monday it sent warning letters to more than a dozen companies the agency said are "illegally selling" products that claim to prevent or cure Alzheimer's disease.

Many of these products, which are often sold on websites and social media platforms, "have not been reviewed by the FDA and are not proven safe and effective to treat the diseases and health conditions they claim to treat," the FDA said in a press release.

"These products may be ineffective, unsafe and could prevent a person from seeking an appropriate diagnosis and treatment," the agency added.

As many as 5.5 million Americans over the age of 65 suffer from Alzheimer's, a progressive disease that often affects memory, thinking and behavior. The number of Americans with the disease is expected to grow, according to the Department of Health & Human Services.

The FDA is responsible for taking action against any misbranded dietary supplement product after it reaches the market.

More than 50 percent of U.S. adults use dietary supplements, the FDA says. Between 2007 and 2016 nearly 800 over-the-counter dietary supplements were found to contain unapproved drug ingredients, according to a study published in October.

The FDA said the products, which FDA Commissioner Scott Gottlieb labeled a "scam," have come in the form of tablets, capsules and oils. The 17 companies who were sent letters have been asked to respond within 15 days.