Kimber Brown of Durango had higher-than-allowed levels of the cough depressant dextromethorphan and Cetirizine, an antihistamine, in her system when she died at her grandmother's home in Hermosa, the tests showed.
Dr. Carol Huser, the La Plata County coroner, said the combination of the two drugs produced a higher level of toxicity in Kimber's body than they would have if used separately.
Huser said it was possible the girl's grandmother either was not aware of the risk of using the two compounds together, or that Kimber had taken the medications on her own.
The Durango (Colo.) Herald said Monday the La Plata County prosecutor was looking into possible criminal charges against Linda Sheets, 59, but Huser determined the cause of death to be accidental.
"I have no reason to suspect any ill intent," Huser wrote in her report. "The degree of negligence in either measuring an inappropriate dose or leaving medications within reach of a child does not, in my view, rise to the level I require for a certification of homicide."