Dunst, 25, began her film career as a child in "Interview with the Vampire," and went on to star in films like "Marie Antoinette," "Elizabethtown," "The Virgin Suicides" and the "Spider-Man" trilogy.
"She's not doing well," a source close to the actress told People.com. "People were pushing her to go in there, but there was no intervention… She has been partying hard for a while and I'm sure the Heath Ledger thing put people over the edge."
Dunst previously dated Jake Gyllenhaal, Ledger's close friend and "Brokeback Mountain" co-star. Ledger died Jan. 22 from an accidental mix of several prescription drugs.
"She's been crying a lot lately, a lot built up. … Everybody hits that bottom where you feel (so) scared that that one heavy night of partying can really wake you up. It's good she's getting herself help."
People.com said it could not reach Dunst's representative for comment.