The actress's passing was due to the toxic effects of fentanyl and additional substances, as confirmed by the Medical Examiner.

In a recent development, the Greenville Police Department has concluded their investigation into the death of actress Hayden Panettiere, ruling it as accidental. The cause of her demise was attributed to the toxic effects of fentanyl and other drugs, as indicated by the Medical Examiner's report 123.
Panettiere, who was 36 years old at the time of her passing, was discovered unresponsive in a South Carolina residence on August 16. The Medical Examiner's findings revealed no signs of trauma contributing to her death, highlighting instead the significant role played by the presence of multiple substances in her system 123.
According to reports, the actress's system contained not just fentanyl, but also methocarbamol and quetiapine. Additionally, the Medical Examiner noted the presence of 4-ANPP, Alprazolam, Methocarbamol, and Quetiapine, further complicating the picture of what led to her untimely end 13.
Brian Hickerson and Zack Hickerson were reportedly with Panettiere when emergency services arrived, adding a personal touch to the tragic event 13. The Drug Enforcement Administration (DEA) was also involved in the investigation, indicating the complexity and potential significance of the case 2.
Likely accurate — probability of truth 85%, weighted across 10 claims from 3 independent outlets.