The family said in a statement that toxicology tests showed "alcohol was present" in the singer's body but it hasn't yet been determined if it contributed to her death.
The 27-year-old soul diva, who had battled drug and alcohol addiction for years, was found dead in her London home on July 23, and an initial post-mortem failed to determine the cause of death.
A statement released by spokesman Chris Goodman on the family's behalf said "toxicology results returned to the Winehouse family by authorities have confirmed that there were no illegal substances in Amy's system at the time of her death."
It said the family awaited the outcome of an inquest that is due to begin in October.
Winehouse's father, Mitch, has said his daughter had beaten her drug dependency three years before her death, but he admitted she was still struggling to control her drinking after several weeks of abstinence.
Mitch Winehouse told mourners at the singer's July 26 funeral that she had said to him, "'Dad I've had enough of drinking, I can't stand the look on your and the family's faces anymore."'
The Winehouse family announced plans to establish a charitable foundation in the singer's name to help people struggling with addiction - although Mitch Winehouse has said the plans are on hold because someone else had registered the name Amy Winehouse Foundation.
In her short lifetime, Winehouse frequently made headlines because of drug and alcohol abuse, eating disorders, destructive relationships and abortive performances.
Her health often appeared fragile. In June 2008 and again in April 2010, she was taken to hospital and treated for injuries after fainting and falling at home.
Her father said she had developed the lung disease emphysema from smoking cigarettes and crack, although her spokeswoman later said Winehouse only had "early signs of what could lead to emphysema."
She turned her tumultuous life and personal demons into songs such as "Rehab," from her Grammy-winning album "Back to Black."
Her death prompted an outpouring of emotion from fans - many of whom left flowers and offerings outside her house in north London's Camden neighborhood - and from fellow musicians.
Her final recording, a duet with Tony Bennett on "Body and Soul," is due to released next month as a charity single.
In Britain, inquests are held to establish the facts whenever someone dies violently or in unexplained circumstances. Winehouse's inquest is due to begin Oct. 26 in London.