Essendon players have struggled to explain why they omitted to mention they had been injected with substances including Thymosin when they were questioned by drug testers during the 2012 season.

Several of the seven past and present Bombers players summoned last month to appear before the Court of Arbitration for Sport were cross-examined as to why — if they believed they were taking legitimate drugs — they neglected to mention those drugs and the injections.

Banned Essendon captain Jobe Watson was among the players to meet. Credit:Getty Images

The matter was also referred to in the closing arguments with CAS panel chairman, the British Queen's Counsel Michael Beloff, referring to the omission on a number of occasions. The three-man panel of independent arbitrators indicated they found the omissions troubling.

A number of Essendon players were asked to give urine samples by the Australian Sports Anti-Doping Authority testers during under the AFL's drug-testing regime. While most of those players detailed other drugs they had taken such as various pain killers and other supplements, the so-called "good Thymosin'' or Thymomodulin was not mentioned — nor the repeated injections — by some players who have since stressed they believed those drugs to be legal under the rules of international sport.