The 'Born This Way' singer's old pal is preparing to release his tell-all book, Rivington Was Ours: Lady Gaga, The Lower East Side, And The Prime Of Our Lives, which will lift the lid on his days living in New York with Gaga.
Due to hit bookshelves on 27 August, the book is thought to be an expose of Gaga's former cocaine addiction and hard-partying ways.
An excerpt from the book provided to Radar Online says Gaga allegedly told Sullivan: “I used to do bags and bags of [cocaine]. Mostly when I was getting ready to go out, while I was putting on my makeup.”
Meanwhile on Tuesday, Gaga was named the top earning celebrity under 30 years old, according to Forbes magazine.
The singer made an estimated $80 million (£52 million) in the last 12 months, despite cancelling part of her tour to have hip surgery.
Forbes claims that Lady Gaga would have earned even more had she not had to cancel part of her world tour in February.
See the full Forbes list below: Top 10 earning celebrities under 30
Lady Gaga - $80 million (£52 million)
Justin Bieber - $58m (£38m)
Taylor Swift - $55m (£36m)
Calvin Harris - $46m (£30m)
Rihanna - $43m (£28m)
Katy Perry - $39m (£25m)
Jennifer Lawrence $26m (£17m)
Adele - $25m (£16m)
Kristen Stewart - $22m (£14m)
Taylor Lautner - $22m (£14m)
Justin Bieber is in second place again this year with $58m (£38m).