The coroner in Park County, Mont. said on Wednesday that Kidder "died as a result of a self-inflicted drug and alcohol overdose".
Kidder is best known for playing intrepid reporter Lois Lane opposite the late Christopher Reeve in 1978's Superman.
In a statement to the Associated Press, Kidder's daughter Maggie McGuane said the family decided to make her mother's suicide public because "it's important to be open and honest about the suicide so there's no cloud of shame".
Regarding her family life, Kidder got married three different times throughout her life and has one daughter, Maggie McGuane.
It's noted that Margot Kidder struggled with mental illness throughout much of her life, which was made worse in a auto accident in 1990 that left her in debt and wheelchair bound for near two years. "Knowing how many families in this state go through this, I wish that I could reach out to each one of them". Kidder also had notable roles in the original Black Christmas and Amityville Horror. Kidder also appeared on the Smallville TV series, and she most recently appeared on The Neighborhood as Maggie. 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.
Kidder and other environmentalists protest outside the White House in Washington in August 2011.
"They are a stigma and recall times when it was thought those with a disorder were possessed by the devil", she told the Edmonton Journal.
"In her last months, she was herself - same kind of love, same kind of energy", Kesich said.
Kidder was working until just prior to her death. She wants to avoid any "cloud of shame".
If you're anxious someone you know may be at risk of suicide, you should talk to them about it, says the Canadian Association for Suicide Prevention.
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