The 32-year-old Clueless star's December 20 death from pneumonia was accidental, assistant chief coroner Ed Winter said.
It was likely her death could have been prevented if she had seen a doctor, he said, but he noted that she did have an appointment with a physician a few days after she died.
Prescription drugs and anaemia also played a role in the death, the coroner's office said.
Mr Winter would not say what types of drugs were involved and said further details would be released in several weeks when the report was completed.
He also said there was no evidence of wrongdoing.
Murphy died after collapsing in her Hollywood Hills home. Her husband, Simon Monjack, and her mother, Sharon Murphy, have said the actress did not abuse prescription medications or have an eating disorder.
They told investigators she had been experiencing flu-like symptoms in the days before she died.
Mr Monjack said last month that his wife took several prescriptions, including an anti-seizure drug, and had been taking over-the-counter Robitussin for her recent illness.
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