WASHINGTON -- The Food and Drug Administration is cracking down on more than a dozen companies that market illegal treatments for diabetes, ranging from bogus dietary supplements to prescription drugs sold online without a prescription.

All of the products aim to cash in on the country's diabetes epidemic, which affects nearly 26 million Americans. Regulators worry that consumers who buy such unapproved products could put off getting legitimate medical care, which could exacerbate heart disease, kidney failure and other deadly complications.

The FDA sent warning letters to 15 companies, both in the U.S. and abroad, ordering them to stop selling diabetes treatments which violate U.S. drug laws.

"Consumers should exercise caution before using products claiming to be herbal or all-natural alternatives to FDA-approved prescription drugs," the agency said in a statement Tuesday.