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.

Three of the products targeted are marketed as "natural" supplements, but actually contain unlisted pharmaceutical ingredients. For example, Diexi, which is sold as a traditional Indian "herbal formula," actually contains metformin, the most common prescription drug used to treat diabetes. The product is sold by Amrutam Life Care, of Surat, India.

"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. "These products should be considered unsafe and should not be used."

Other products include genuine dietary supplements that make unproven claims to treat or prevent diabetes. For example, Diabetes Daily Care is a capsule-based supplement containing cinnamon extract and other herbs. Its manufacturer, Nature's Health Supply Inc., claims it "safely and effectively improves sugar metabolism."

Under U.S. law, only FDA-approved medicines are permitted to make claims for treating or preventing disease.

Other companies targeted by the FDA run online pharmacies that sell prescription drugs for diabetes without a prescription. The FDA issued a warning letter to www.bestcheapmedsonline.com for marketing unapproved versions of diabetes drugs like Januvia, from Merck & Co. Inc.

The FDA warns patients against buying prescription medications on the Internet. Only 3 percent of online pharmacies actually comply with all U.S. pharmacy laws, according to a review by the National Association of Boards of Pharmacy.

People with diabetes are unable to properly break down carbohydrates, either because their bodies do not produce enough insulin or because they've become resistant to the hormone, which controls blood sugar levels. These patients face higher risks of heart attacks, kidney problems, blindness and other serious complications.

Many diabetics require multiple drugs to control their blood sugar levels.

The U.S. market for prescription diabetes drugs is the largest in the world, with sales of $22 billion last year. Sales have ballooned more than 60 percent in the last four years from $13.6 billion in 2008, according to health data firm, IMS Health.

The FDA said it has not received any reports of injury or illness connected with the products, but is taking action as a precautionary measure.

The FDA sent the warning letters to the companies last week, but posted them online Tuesday morning. The letters gives each company 15 business days to reply and explain how they will come into compliance with U.S. law. FDA warning letters are not legally binding, but the agency can take companies to court if they are ignored.