Watson, 29, tested positive for Clomifene – a hormone modulator – on 28 September 2019.
Less than a month he confirmed he would retire from boxing.
UK Anti-Doping charged and suspended Watson on 4 December.
“Mr Watson admitted the anti-doping rule violation and explained that the adverse analytical finding was the result of self-medicating an apparent medical condition,” UK Anti-Doping said.
“As he admitted that he had taken the drug without delay, Mr Watson’s two-year ban started on 28 September 2019 and will expire at midnight on 27 September 2021.”