Brittany Murphy was ill with flu-like symptoms in the days before her death, the Los Angeles coroner's office said.
The 32-year-old star of films such as "Clueless" and "8 Mile" died Sunday morning after collapsing at her Hollywood Hills home.
Paramedics tried to revive her, but she was pronounced dead at Cedars-Sinai Medical Centre. Police said there was no evidence of foul play.
The coroner's office said prescription medications were taken from her home.
Assistant Chief Coroner Ed Winter said they will conduct a post-mortem examination to try to determine what killed the actress, and said her death appeared to be from natural causes. He said the illness, reported to officials by her family, could have contributed to her death, but it will be weeks before a final determination is made.
Toxicology tests will be performed, and officials will contact her personal physician to get a better sense of Murphy's medical history, Winter said.
Neighbour Clare Staples said she saw firefighters working to resuscitate the actress who was on a stretcher.
Murphy's husband, wearing pyjama bottoms and no shoes, appeared "dazed" as firefighters tried to save her, Staples said. "It's just tragic," she added.
Police investigators went to Murphy's home to launch a standard death investigation. Detective Wendi Berndt, who supervises the Los Angeles Police Department's homicide unit in Hollywood, said there was nothing to indicate suspicious activity.