Howlett, along with 33 other Essendon players, was found guilty of using a banned performance-enhancing substance, thymosin beta-4, as part of Essendon's sports supplements program during the 2012 season. He and his teammates were initially found not guilty in March 2015 by the AFL Anti-Doping Tribunal,[2] but a guilty verdict was returned in January 2016 after an appeal by the World Anti-Doping Agency. He was suspended for one season which, with backdating, ended in November 2016; as a result, he served approximately fourteen months of his suspension and missed the entire 2016 AFL season.[3]
Howlett was delisted at the conclusion of the 2017 AFL season.[4] He returned home to WA and signed with the club he was originally drafted from, Peel Thunder in the WAFL, for the 2018 season.[5] In 2019, Howlett was named captain of the club, taking over from dual premiership captain Gerald Ugle.[6]
In Round 9 of the 2021 WAFL season Howlett played his 100th[7] game for Peel Thunder against Swan Districts before announcing his retirement from WAFL footy at the end of that season.