"She has passed her random drug tests. There's been more than one. They came up negative," People quoted Trope, as saying.
The attorney also revealed that the 25-year-old singer has also been holding sessions with, and taking advice from, a parenting coach.
Asked if Spears has been cooperating with her court-mandated parenting coach, Trope said, "As far as I know, there have been no complaints."
He added that the singer's chances of regaining shared custody of her sons are "good, based on the overall circumstances of the case."
Lawyer Mark Vincent Kaplan, who is representing Kevin Federline, Spears' ex-husband, in the custody battle, was not immediately available for comment.
Last month, Los Angeles Superior Court Commissioner Scott Gordon ruled that Spears is a "habitual, frequent and continuous" user of alcohol and controlled substances, and ordered her to undergo twice-weekly random testing.
Two weeks later, the singer was temporarily deprived of custody, with Federline, retaining the physical custody of the boys for now.
Spears has been granted monitored visitation rights with her little boys and must complete a list of orders from the judge overseeing her custody case before her next hearing on Oct. 26.