Dr. Daniel Ubani accidentally killed David Gray in February 2008 by giving him 10 times the normal recommended maximum dose of diamorphine, The Guardian reported Monday.
Ubani was working for Doctors on Call when he treated Gray, who suffered from kidney stones and renal colic, at his home in Manea in Cambridgeshire.
Ubani, a German national who had worked beyond his shift and was covering for other doctors, allegedly admitted he was exhausted when he visited Gray, Britain's Care Quality Commission said in releasing its findings.
"This is a deeply disturbing case and one that must be thoroughly looked into. We have to ensure any lessons are learned," said Christine Braithwaite, a commission spokeswoman.
Ubani was given a suspended prison sentence, fined $6,600 and prohibited from working in the United Kingdom, the Guardian reported, noting Ubani now practices in Witten, Germany.
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