Essendon may call in police to investigate the origin of a letter purportedly from the World Anti-Doping Agency (WADA), which gave the all-clear to use an anti-obesity drug on its players.

The Herald Sun says the letter, presented to the club by sports scientist Stephen Dank, was a dupe.

Essendon - the AFL club at the centre of the drugs-in-sport scandal - told the Herald Sun in an email that it did not give permission to Dank to use the drug.

Last week WADA confirmed that anti-obesity drug AOD 9604 was a banned substance.

The paper says Essendon believes it may have been tricked into using the drug, and will seek legal advice in a bid to clear its players of doping sanctions.

AAP