It comes after Dr Gabriel Okey worked without a licence for six weeks in 2016, including a period at HMP Bedford.
The Medical Practitioners Tribunal Services found he knowingly worked even though his licence had been revoked three months previously and so he had "abused his position of trust". The panel suspended him for a year in January 2018.
But a fresh tribunal has ruled that he can now return to work.
Panel chair Leighton Hughes said Dr Okey had shown "meaningful reflection" and full insight into his misconduct, and believed that the risk of repetition is low.
He said: "[The suspension] served to demonstrate to the public and the profession that misconduct of this nature was unacceptable.
"The tribunal was satisfied that this purpose has been met by the period of 12 months suspension from the medical register, and it is no longer necessary for this tribunal to make a finding of impairment on this basis."
During his suspension, Dr Okey prescribed methadone, antibiotics, antidepressants and asthma medication, among other drugs.