Prince's ex claims he lived on a "cocaine diet" and used to "shake all night" while he slept.
Charlene Friend insists she never witnessed the late singer - who died in April aged 57 from an overdose of narcotic fentanyl - taking drugs, but has said the star's half-brother Duane Nelson, who died in 2013, said the Purple Rain hitmaker used to take illegal substances.
She said Duane once told her: "'We call it the cocaine diet.'
"It was shocking to me to hear that. He hid it so carefully. I never saw him take even an aspirin.
He seemed so health-conscious, always taking vitamins like echinacea and goldenseal."
The 44-year-old interior designer - who dated Prince for two years after meeting him in 1989 - also claimed the star went up to five days without sleep but when he did get some shut eye he covered the windows in aluminium foil to make sure the room was pitch black and turned the heat up to 80 degrees.
She added to People magazine: "He would sometimes stay up for five days at a time without sleep, food or even water, going nonstop back and forth to the studio.
"I had to sneak in catnaps to keep from passing out from exhaustion. The first time that happened, I asked him, 'How do you do this?' He would be shaking all through the night.
"The first time that happened, I asked him if he was having a nightmare and he said, 'No. I was dreaming music.' Even in his sleep, the music just poured out of him."
Charlene also admitted she heard speculation from mutual pals saying Prince was "taking pills to come down" from the high he got from performing.
She said: "Nothing could match the highs he got from his shows. Coming down afterwards made him feel empty. Perhaps drugs filled that void."
In 2003, Charlene sued Prince for emotional distress and defamation, claiming she was psychologically abused during their relationship.
She claimed in court documents: "Prince informed me he took sexual relations very seriously. He believed he was the Messiah and if you engaged in sex with him, you became one with him.
"He would have me dress in his clothing at his whim. His staff were not allowed to look at me and I was not allowed to look at them."
But Prince won the case after his legal term proved he was the victim of a "spiteful lawsuit".