Kildare GP Dr Denis O'Dwyer has been found guilty on four counts of professional misconduct and two of poor professional performance.
A decision on any sanction will be made by the full Medical Council at a later stage.
He faced ten allegations of professional misconduct or poor professional performance in relation to prescribing tranquillisers to a teenager who had substance misuse issues.
The Medical Council's fitness-to-practise inquiry examined the actions of Dr O'Dwyer in prescribing benzodiazepines to Patient X, a minor, for 18 months between July 2009 and January 2011.
The inquiry heard that he did not inform the patient's parents and the substance misuse service Patient X was attending.
JP McDowell, solicitor for the Medical Council's chief executive, said benzodiazepines should usually only be prescribed for four weeks because they are addictive.
Patient X told the inquiry that in 2009, at the age of 15, he visited Dr O'Dwyer and told him he could not sleep and was a recovering heroin addict.
He did not tell Dr O'Dwyer he was in drug support programmes and he never paid the GP.
Patient X overdosed on the tablets in August 2009 and was treated at Naas General Hospital.
He said Dr O'Dwyer did not conduct a major medical examination, but claimed there were times when they argued about the medicines.
Patient X said he got a prescription around once a month and sometimes these were pre-signed and handed out by the receptionist.
The inquiry heard that Patient X was banned from the practice in 2011.
Dr O'Dwyer told the inquiry that he did not wish to give evidence or call witnesses.
He said he found huge problems in getting patients into drug support schemes, due to long waiting lists.
An expert witness in general practice, Professor Colin Bradley of University College Cork, told the inquiry that benzodiazepines should only be used in small doses and for the shortest time possible, given the risk of dependence.
Prof Bradley said he would expect a GP to have taken a proper medical history of a patient at the outset.
To have taken none was "a serious falling short" in the standard expected of a doctor, he said.