In a statement released to ET by Park County coroner Richard Wood, it was announced that Kidder "died as a result of a self-inflicted drug and alcohol overdose".
Margot Kidder's death has been ruled a suicide. She became paranoid and disappeared for four days until she was found in a stranger's backyard and taken to Olive View Medical Center by police.
Maggie McGuane told the Associate Press that she knew her mom died from suicide after she went to her mom's home on May 13, the day her mom died.
She had been diagnosed with bipolar disorder and, in 1996, she suffered a public manic episode and nervous breakdown.
Now, nearly three months after Kidder's body was found by a friend in her Livingston, Montana home, a coroner there has ruled her death a suicide. Her obituary at the time said simply that she passed away at her home, and does not mention her cause of death.
Joan Kesich, a longtime friend who found Kidder's body, said she had been fearless and always spoke the truth, regardless of the consequences. She wants to avoid any "cloud of shame".
In releasing the cause of death her family hoped to erase stigma around suicide and encourage others to seek help.
The actress had publicly struggled with mental-health issues.
She says it's a unique sort of grief and pain and that she would like to reach out to every family who is suffering through a loss by suicide.
She later appeared in small films and television shows until 2017, including R.L. Stine's the Haunting Hour.
Both Kidder and co-star Christopher Reeve, who played Superman, were relative unknowns when they got their leading parts, and neither saw many major roles afterwards.