A toxicology examination detected no amounts of cocaine, marijuana, hallucinogenic drugs or prescription medications, the Los Angeles County Coroner's office said in a report.
Lewis, 28, was found dead in the driveway of a home he had been renting in Los Angeles and his 81-year-old landlady was found dead inside.
Investigators initially suspected Lewis was under the influence of drugs or alcohol because he had spent time in rehab and had a criminal drug past. The actor had been released from jail five days prior to his death.
Authorities ruled Lewis' death an accident from partial strangulation and head trauma after the actor fell 5m from an elevated terrace onto a paved driveway.
"We do not have definitive evidence that (Lewis) intended to kill himself," Los Angeles County medical examiner James K. Ribe wrote in the autopsy report. "We also do not at this time have any evidence that he was pushed."
Detectives believe Lewis may have killed his landlady, Catherine Davis, but said the motive was unknown.
Lewis had pleaded guilty in August to assault with a deadly weapon. He also pleaded no contest to an attempted burglary in a separate case. He was sentenced to jail in both cases.
Lewis played Kip "Half Sack" Epps, an Iraq War veteran, in the first two seasons of FX television drama "Sons of Anarchy" about an outlaw motorcycle club. The Los Angeles-born actor previously had recurring roles in the TV shows "Boston Public" and "The O.C."