The 23-year-old, who tested positive for pseudoephedrine and was suspended by ASADA earlier this month, failed to convince a Perth hearing his overuse of the substance was accidental.
Casserly had been training with the Tigers in the hope of winning a spot on the rookie list.
Perth magistrate Paul Heaney was unconvinced by Casserly's defence that use of cold and flu medication had caused the positive test, conducted after his side's one-point WAFL grand final triumph last year.
"Having heard the evidence at the tribunal hearing ... I am not persuaded to a comfortable persuasion of the absence of intent to enhance his sporting performance," Heaney said after the ASADA hearing.
"I am satisfied to the requisite standard that ... Casserly did have the intent to enhances his sporting performance.
"I am satisfied that the high reading of pseudoephedrine found in ... Casserly's urine sample, indicated that his use of the substance went beyond the therapeutic to the enhancement of his sporting performance and that is was his intent for it to do so."
Heaney said he Casserly's period of ineligibility would begin from October 18 last year.
Casserly was on Richmond's list from 2006-08 before spending the past two seasons playing for Swan Districts in the WAFL.
He was invited to train with the Tigers alongside other hopefuls Mitch Keddell and Liam Corrie vying for the rookie place on the 2011 list.
Casserly was pencilled in to play for the Tigers in their pre-season hit out against the Indigenous All-Stars in Alice Springs before the game was cancelled due to poor weather.
He is the second WAFL player to recently test positive to a performance-enhancing drug after East Perth's Dean Cadwallader admitted to using banned steroid nandrolone last season.
An AFL spokesman said the League supported the decision made by ASADA and the anti-doping code.
