"One time David and Iggy (Pop) were looking for some blow," the "Call Me" hitmaker writes. "Their connection in New York had suddenly died and they were out."
"A friend had given me a gram, but I had barely touched it. I didn't care for coke too much - it made me jittery and wired and it affected my throat. So I went upstairs with my vast quantity of cocaine and they just sucked it right up in one swoop."
"After they did the blow, David pulled out his c**k - as if I were the official c**k checker or something. Since I was in an all-male band, maybe they figured I really was the c**k-check lady."
Debbie was not offended by David exposing himself, writing: "David's size was notorious, and he loved to pull it out with both men and women. It was so funny, adorable and sexy."
Speaking to the Sunday Times, she adds that it's a shame the "Starman" musician, who died in 2016 after a battle with cancer, is no longer around to confirm the story.
"It was very funny... I didn't touch it," she explains. "But I did think, well - very nice. I don't know, it's too bad you can't ask him."
"Face It: A Memoir" is available from Tuesday, October 01.