Dr Michael Stevenson worked as a GP in Seascale
A doctor who gave a patient six times the proper dose of a painkiller has told a General Medical Council panel he "bitterly regrets the error."
Dr Michael Stevenson, 57, of Cumbria, said he thought he had given a 59-year-old builder 5mg of diamorphine in 2005, but had actually administered 35mg.
The panel heard the man stopped breathing and needed an antidote.
In 2007, the 57-year-old GP admitted manslaughter after accidentally giving a female patient a fatal overdose.
He was given a suspended jail sentence for the mistake which killed Marjorie Wright, of Workington, in January 2005.
The fitness to practise hearing was told Dr Stevenson, from Bootle, near Millom, visited the builder, who was suffering with back pain, just an hour after seeing Mrs Wright.
Dr Stevenson said: "As far as I can remember, I saw 5mg. How I came to do that I cannot say. It may be that I was expecting to see that figure.
"Clearly I bitterly regret the error. I go over it in my mind and try to find an explanation. It has had a big effect on me.
"I have never ever made an error of that magnitude in my life."
The man stopped breathing after the injection and went blue in the face.
He went on to make a full recovery, after paramedics resuscitated him and administered the antidote.
The GP, who worked at a practice in Seascale, admits he later recorded in his notes that he administered 5mg of the drug.
He is appearing before the panel in Manchester to face claims in relation to four patients he treated with painkilling drugs while working for out-of-hours GP service Cuedoc.
He denies his fitness to practise is impaired because of his alleged actions.
He remains on an interim suspension order, meaning he cannot work as a doctor, and has previously vowed never to practise again.
The hearing continues.