Dr Archibald Alexander, from Croydon in Surrey, was found guilty of serious professional misconduct at a General Medical Council hearing in London. He has 15 years experience with the prison medical service.
The GMC disciplinary committee found he had been "irresponsible" in prescribing fatal doses of the heroin substitute to Carl Owen and David Davies, two inmates at Brixton Prison in south London in 1994.
The committee chairman, Dr Jeremy Lee-Potter, accepted that Mr Owen and Mr Davies tried to fool the 63-year-old doctor into believing they were suffering withdrawal symptoms.
"A conscientious assessment of the... patient's condition is a fundamental aspect of good clinical practice and is essential before initiating treatment," said Dr Lee-Potter.
Dr Alexander treated both men with 50 millilitres of methadone and a twice-daily dose of valium for heroin withdrawal without properly checking their medical history, said Michael Mansfield QC, representing the families.