The Australian Sports Anti-Doping Authority (ASADA) is defending its handling of the Essendon case amid fresh reports on Wednesday about the timing of the listing of the banned drug taken by some players.

The Herald Sun newspaper is reporting the drug Thymosin Beta-4, which was administered to Essendon players that led to their AFL suspension in 2016, was only listed as a banned substance several months after the injections were stopped.

Some 34 Bombers players were banned for 12 months after a Court of Arbitration for Sport ruled the players were injected several times with Thymosin Beta-4.

The Herald Sun reports the drug was only deemed to be logged as prohibitive by ASADA on February 4, 2013 - months after the club had stopped injecting the players.

"An ASADA official ran a check on the public site for "research" at 10.34am and no flag was generated for Thymosin Beta-4," the newspaper reported.

"Another check on Thymosin Beta-4 just over two hours later at 12.59pm by an ASADA staffer listed it as 'banned in sport'.

"The status update came on the same day AFL boss Andrew Demetriou called Essendon chairman David Evans about a secret investigation into the club's supplements program, prompting the Bombers to 'self-report' to ASADA."

But ASADA said the timing of the logging of the drug on its former checking tool known as Check Your Substance was irrelevant.

"ASADA strongly denies allegations of evidence manipulation reported in the media," the organisation said in a statement.

"The World Anti-Doping Agency (WADA) has sole responsibility for setting and determining prohibited substances. It is not possible to manipulate whether or not substances are prohibited.

"Thymosin Beta 4 was included on the 2010 prohibited list by WADA .... Thymosin Beta 4 has never been approved for human use.

"Whether a substance was on ASADA's former Check Your Substances tool has no bearing on its status as a prohibited substance on WADA's prohibited list.

"It is impossible to list every substance that may be used for performance enhancement in such a tool, particularly those not approved for use by humans."

The supplements saga led to a number of senior Essendon figures leaving the club, including chief executive Ian Robson, chairman David Evans and his replacement Paul Little.

Then-coach James Hird was banned by the AFL for 12 months in 2013 when the AFL fined the club $2 million and banned the Bombers from competing in that year's finals series.

Hird returned after the conclusion of his ban but later resigned.

Sports scientist Stephen Dank, who oversaw the supplements program, was in 2015 found guilty by an AFL Tribunal of trafficking in a number of illicit supplements.

The tribunal banned Dank from any association with the AFL for life.

Australian Associated Press