Nathaniel Mendez-Laing says he wants to "leave the past behind" after serving a suspension for cocaine use.
The Football Association (FA) has revealed the 28-year-old served a three-month ban after a positive drugs test administered on 4 July, 2020.
Cardiff released Mendez-Laing in September 2020 for what the club termed an "alleged breach of contract".
The FA said the now Middlesbrough wing admitted taking cocaine "in a context unrelated to sporting performance."
Mendez-Laing was originally charged and provisionally suspended for breaching anti-doping regulations on 14 August, 2020 and could have faced a longer ban.