The cause of death for Margot Kidder, best known for portraying Lois Lane in Superman: The Movie opposite Christopher Reeve, has been revealed.
On Wednesday, coroner Richard Wood of Montana’s Park County Coroner’s office confirmed to the Associated Press that Kidder’s death was “a result of a self-inflicted drug and alcohol overdose.”
PEOPLE confirmed the actress died on May 13 at her home in Livingston, Montana. She was 69.
A representative for the Park County Coroner did not immediately respond to PEOPLE’s request for comment.
WATCH: Superman’s Lois Lane, Margot Kidder, Dies at Age 69
After Kidder’s cause of death was revealed, the star’s only daughter, Maggie McGuane, told the AP, “It’s a big relief that the truth is out there. It’s important to be open and honest so there’s not a cloud of shame in dealing with this.”
Kidder’s struggle with bipolar disorder, which became highly publicized after a 1996 manic episode left her homeless for a time.
Kidder told PEOPLE five months after the incident that the root of most of her problems — which include “mood swings that could knock over a building” — was bipolar disorder. She was first diagnosed with the condition by an L.A. psychiatrist eight years prior, but she refused to take the recommended prescription of lithium, the recommended treatment.
After receiving treatment, Kidder went on to become a lifelong advocate for mental health awareness.
Kidder began acting in the late 1960s, but rose to fame after 1978’s Superman: The Movie. She went on to reprise the part in the movie’s three sequels.
She also starred as Kathy Lutz in The Amityville Horror (1979) and appeared in movies such as Black Christmas (1974) and Heartaches (1981).
Kidder acted as producer and starred as Eliza Doolittle in a 1983 adaptation of Pygmalion with Peter O’Toole for Showtime.
If you or someone you know is considering suicide, please contact the National Suicide Prevention Lifeline at 1-800-273-TALK (8255), text “help” to the Crisis Text Line at 741-741 or go to suicidepreventionlifeline.org.