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.