Named only as Doctor D, the man said he sold drugs to him between 1984 and 2008.
Doctor D told the Mail Online: "I first met Prince in 1984 while he was filming the movie Purple Rain and he was already majorly addicted to opiates - I didn’t hook him on drugs.
"He was already a really heavy user.
"I’d say his habit was costing him about $200-300 a day, but that didn’t matter to him as he had plenty of money - he never ran out."
The interview paints a picture of an artist riddled with anxiety.
He added: "He needed the drugs because he was so nervous - he could be nervous in a room with just five people in it.
"He wasn’t really a party guy, he was doing these drugs so he could feel at ease around people.
"He was scared to go out in public, he was scared to talk to people and didn’t like to go on stage. He had the worst case of stage fright I’d ever seen.
"A lot of performers rely on drugs to make them feel confident on stage, but he was by far the worse.
"I remember when he was filming Purple Rain he was buying a lot of drugs - I think it was nerve wracking for him to have to perform in front of cameras and people every day so he needed the drugs."
Read the full interview here.
Prince, 57, was found dead at his Paisley Park estate in Minnesota.
Prince had been hospitalised the week before, with his private plane having to make an emergency landing in Illinois so the star could be treated on the way back from a gig in Atlanta. He later returned to home at Paisley Park and was able to appear on stage the following day.
At the weekend, he made a final public appearance at a show, where he didn’t perform, but told the crowd, “Wait a few days before you waste any prayers.”
According to TMZ, authorities had responded to a medical emergency at his Paisley Park studio on Thursday morning. The Carver County Sheriff’s department in Chanhassen, Minnesota told TMZ that there had been a fatality, and they were notifying next of kin.
Prince - real name Prince Rogers Nelson - was one of the world’s most successful musicians, having sold more than 100 million records in his 30-year career.
He was perceived as arguably the most musically gifted of the pop stars who came to prominence in the mid-1980s. He could perform with huge numbers of instruments, and was always spoilt for choice as to what to include on his albums, because he had written hundreds of songs in preparation for each one.
In his stunning, unique career, he won seven Grammy Awards, a Golden Globe and an Oscar - for the score to ‘Purple Rain’. The very first year he was eligible, he was inducted into the Rock and Roll Hall of Fame. The prestigious Rolling Stone’s poll ranked him at number 27 on its list of the 100 Greatest Artists of All Time.
Born in Minneapolis, he was writing songs from the age of seven. He was only 20 when his album ‘Prince’ went platinum with the songs ‘Why You Wanna Treat Me So Bad?’ and ‘I Wanna Be Your Lover’.
Influenced by funk, dance and rock music, he nevertheless tapped into a mainstream pop vein, with his worldwide pop smash ‘Purple Rain’, which served as a soundtrack to his film debut of the same name. From 1984, his band were termed ‘The Revolution’ and a musical myth was created.
Following the disbanding of the Revolution in 1986, Prince built his reputation as a solo artist with ‘Sign o’ The Times’.
In 1993, he was sufficiently famous to change his name to an unpronounceable symbol as a mark of his artistic independence:
This was in 1993, and for the next seven years, he released albums at an ever-faster rate to remove himself from his Warners label contract. Between 1994 and 1996, he produced five albums in total, in 1998 he signed to Arista, and by 2000 he was back to being Prince.
His prodigious output never waivered. This century, he released a further 15 albums. His latest, HITnRUN Phase One, which was first released in September 2015.
He had performed in the UK the year before, including two sell-out performances at Camden’s Electric Ballroom and the Shepherds Bush Empire.