REELING from fan attacks after landing the lead role in “Fifty Shades of Grey,” a fragile Dakota Johnson is in danger of slipping back into substance abuse, friends fear.

Readers of the racy best-seller have bombarded the Internet with posts protesting the selection of the 23-year-old beauty to play Anastasia Steele in the film version.

The daughter of Melanie Griffith and Don Johnson has been “deeply hurt” by the nasty backlash after being tapped to portray the virginal coed who is introduced into the kinky underworld of bondage and discipline by sexually sadistic industrialist Christian Grey, played by “Sons of Anarchy” hunk Charlie Hunnam.

Incredibly, an online petition to cast two other actors quickly gathered more than 79,000 signatures.

“After she was chosen for the movie, Dakota was on top of the world,” said a friend. “But when she became the target of irate fans of the book, it freaked her out.

“With all the pressure, she could easily slip and fall off the wagon.”

As The ENQUIRER reported in December 2007, Dakota’s heartbroken parents – who’ve had their own problems with substance abuse – sent their hard-partying daughter to rehab.

Now friends fear the ugly fan reaction to Dakota could reverse her progress.

 Los Angeles addiction expert Dr. Gregory Smith has not treated Dakota but told The ENQUIRER: “The number one reason addicts relapse is stress.”

Even Melanie has rallied to the defense of her daughter, who most recently starred in the short-lived FOX sitcom “Ben and Kate.”

Responding to a particularly nasty tweet, Mama Melanie fired back: “Wow! You’re quite the b**** aren’t you?”