Concerns that Essendon players were given a banned performance-enhancing drug have strengthened with the emergence of an invoice showing the club was billed for a "Thymosin peptide".

The invoice provides a paper trail between the club and the possible provision to players of Thymosin beta 4, a substance banned by the World Anti-Doping Agency since at least 2011 for its performance-enhancing effect and viewed more seriously than the failed anti-obesity drug AOD9604.

Essendon coach James Hird. Credit:Justin McManus

Fairfax Media understands that Essendon players have been unable to explain to anti-doping investigators whether the club's 2012 supplements program involved Thymosin beta 4, popular with body builders because it aids muscle recovery, or a benign version, known as Thymosin alpha, used to treat AIDS and cancer patients and deemed by WADA not to have any performance-enhancing effect.

It is understood that player testimony on the issue of Thymosin has been vague, with players unable to specify which type of the drug was taken.