"Her behaviour suggests bipolar disorder," Usmagazine.com quoted Greer.
"The manic episodes with impulsivity and loss of control - the flashing, the head shaving, the hitting the photographer's car with the umbrella - are what we see," she says.
Greer, in the Allure article, points to an incident in June when Britney publicly delivered a letter to her mother Lynne telling her to stay away from her sons Sean Preston and Jayden James.
"Part of manic behavior is a flight from reality and the anger. People who are manic get angry at people who try to take away their high - like her mother, for example - by introducing any element of reality," Greer added.
The article appears in place of Britney's interview, after the singer missed four appointments with the mag.