The U.S. Food and Drug Administration and the U.S. Drug Enforcement Administration have jointly issued warning letters to two websites for illegally selling Adderall, a treatment for attention deficit hyperactivity disorder (ADHD).

Adderall is an FDA-approved prescription drug made of two stimulants amphetamine and dextroamphetamine, and has a high potential for abuse and addiction.

The health agency said the websites selling Adderall without a prescription pose a risk to consumers as the products, while being marketed as authentic, may be counterfeit, contaminated, expired or otherwise harmful.

"Illegal sale of prescription drug stimulants online puts Americans at risk...these particular types of online pharmacies also undermine our efforts to help consumers safely purchase legitimate prescription medicines over the internet," FDA Commissioner Robert M. Califf said.

The warning letters were issued on March 30 to Kubapharm.com and Premiumlightssupplier.com

The companies have 15 business days to respond to the agencies and inform them of the steps being taken to address any violations and prevent their recurrence.

Both the online operators did not immediately respond to Reuters' requests for comment.