Beware of fake versions of ADHD medication  Adderall that are being sold on the Internet,  the U.S. Food and Drug Administration has warned.

The agency issued the warning Tuesday after receiving complaints from the manufacturer Teva Pharmaceutical Industries that the 30 mg tablets, which do not even resemble the authentic version, was being sold online under the company name, FDA spokesperson Shelly Burgess told ABC News.

Adderall is approved by the FDA to treat attention deficit hyperactivity disorders (ADHD) and narcolepsy.

"We issued the warning to warn the public of the risks of buying from internet sellers," said Burgess.

The agency, which conducted lab tests on some of the counterfeits, found the counterfeit version did not contain the correct active ingredients for Adderall.  Instead, it contained acetaminophen and tramadol, which are used to treat acute short term pain.

Since there is no tracking system for rogue websites that sell fake medication, it is unclear how many people have bought the imitations online.

The nationwide shortage of Adderall in pharmacies may have led some to purchase the medication on the Internet. The FDA's website lists many dosages of Adderall manufactured by Teva pharmaceuticals have been in short supply since at least October 2011.

According to the FDA, counterfeit Adderall may be white instead of orange, contain no markings on a tablet, and come in a blister package. There may also be misspellings on the package.

Trustworthy websites are licensed by state board of pharmacy and contain a list of the boards on the site. The site should also have a licensed pharmacist available to answer questions the consumer may have.   Sound websites also require prescription verification before dispensing any pharmaceutical medication.

"If it offers dramatically low prices from market value, this is a red flag," said Burgess.

Anyone who suspects they have purchased the counterfeit version should stop taking the medication and report any side effects to the FDA's MedWatch Safety Information and Adverse Event Reporting Program, said Burgess.

The agency is grappling with how best to regulate rogue online sites that sell fake and potentially harmful medications. The FDA can issue warning letters to the sites highlighting its unsafe practices, but are not able to ensure that it can be stopped.

"These sites can shut down and pop back up under a different name the next day," said Burgess.