The toxicology results did show use of the sleep aid zolpidem, which is marketed under the name Ambien.
An initial examination of his brain also showed no signs of brain damage from his years of football, according the San Diego medical examiner.
Seau, 43, was found dead in his two-story beachfront home on May 2.
His girlfriend called 911 around 9:30 a.m. reporting that Seau was unconscious and bleeding from a gunshot wound to the chest.
When emergency crews arrived, they tried unsuccessfully to revive Seau.
A handgun was found near Seau's body, but he left no note.
His girlfriend, Megan P. Noderer, told police Seau had given no indication he planned to kill himself, according to the autopsy report.
When Noderer returned to the home in Oceanside on May 2 after a morning workout at a gymnasium, she found Seau in bed, dead of a gunshot to the chest from a .357-caliber revolver.
No note or "documents that were suicidal in nature" were found.
The 6-foot-3, 275-pound Seau, a star at Oceanside High, USC and then the NFL, was taking prescription medicine for orthopedic problems but only drank "socially (and) did not smoke, and there was no history of illicit drug use," according to the report.
At the request of the Seau family, brain tissue was sent to the National Institutes of Health for more complete testing.
Brain damage among football players has become a controversial issue in the National Football League, where Seau was a star for the San Diego Chargers and other teams.
An All-American at USC and 12-time NFL Pro Bowl linebacker, Seau played 13 seasons with the San Diego Chargers and three seasons with the Miami Dolphins.
He left the game briefly but then played with the New England Patriots for four seasons before calling it quits for good after the 2009 season.
Seau is survived by two sons and daughter by ex-wife Gina Deboer.