An amputee died after taking a painkiller 50 times stronger than heroin.
The body of Karl Ryan, 47, was found in his bungalow on Smithy Croft, Dronfield, on August 10 last year.
Mr Ryan's inquest, which was held at Chesterfield coroners' court on Thursday, heard he had high levels of fentanyl in his system.
In the last eight months, at least 60 people have died in the UK after taking fentanyl, a highly toxic opioid.
It has been linked to thousands of deaths in America, including that of the singer Prince.
Coroner Peter Nieto read out a statement by Mr Ryan's fiancee, Donna Moore, who discovered his body.
Ms Moore said in her statement: "He had a fentanyl patch in his mouth which he used to help him with constant pain.
"He was happy with life and looking forward to our marriage," she added.
Dr Miles Davidson, a GP at Stubley Medical Centre in Dronfield, said Mr Ryan was already taking a 'very high' dose of fentanyl when he joined the practice in August, 2014, after having his right leg amputated.
According to Dr Davidson, Mr Ryan had 'major pain issues' and was not in a position to be able to take less fentanyl.
Dr Davidson said: "We asked two consultants if we could admit him to hospital to establish what dose he actually needed - but they both said no beds were available."
Mr Ryan was taking less fentanyl by January, 2016, the court heard.
Dr Davidson added: "It looks like there was a long history of him using fentanyl patches orally.
"That's really dangerous - I didn't know this was happening."
Mr Nieto recorded a conclusion of a drug-related death.
He said: "Mr Ryan died as a result of his misuse of fentanyl patches which were prescribed for pain relief.
"On the evidence presented to the inquest, it is clear that at least, in part, his misuse of fentanyl patches was driven by his desire to obtain a psychoactive effect.
"On the evidence, it is likely he had been misusing fentanyl for a number of years."
Mr Nieto offered his condolences to Mr Ryan's loved ones at the conclusion of the inquest.