The coroner did not specify what drugs she took.
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.
Kidder struggled with mental illness through a good part of her life, a situation that was made worse by a 1990 vehicle crash that left her in debt and led to her using a wheelchair for nearly two years. Her obituary at the time said simply that she passed away at her home, and does not mention her cause of death. While living in Montana, she was also an activist for progressive causes and animal rights.
Kidder was working until just prior to her death.
"It's important to be open and honest so there's not a cloud of shame in dealing with this", she continued.
In a statement, the Park County, Montana, coroner's office confirmed to the wire service that the actress "died as a result of a self-inflicted drug and alcohol overdose".
Kidder was married to novelist Thomas McGuane with whom she had a daughter, Maggie.
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.
She had the same role in a number of different movies, including the three sequels afterward: Superman II, Superman III, and Superman IV: The Quest For Peace.
"In her last months, she was herself - same kind of love, same kind of energy", Kesich said. Her film "Robber's Roost" was in pre-production, and the final film she worked on was 2017's "The Neighborhood". "She was really a blazing energy".