The rumor was that she died from the flu because she had been fighting the flu and talking about flu symptoms just days before her death.
The Park County Coroner says Kidder's overdose was self-inflicted.
Her obituary in the New York Times did not specify a cause of death.
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Kidder's daughter said she knew her mother died by suicide the moment authorities took her to Kidder's home. She told the AP she believes it's important to be open and honest about Kidder's suicide so neither her nor her family faces a cloud of shame as a result.
Kidder, who played Lois Lane opposite Christopher Reeve's Superman, was found by a friend in her Montana home on 13 May. She continued to act in many movies and TV shows, most recently the 2017 film "The Neighborhood".
But Kidder had a history of mental illness.
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Kidder also battled bipolar disorder and was homeless for a short time in 1996.
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. Since the 2000s, her acting career had picked back up again and she was starring on Smallville, Brothers and Sisters, the L Word, R.L. Stine's The Haunting Hour, and she was appearing on Broadway. But on a trip to NY she caught the acting bug, and after school she moved to Los Angeles to try her luck. She was born to Kidder and her first husband, novelist Thomas McGuane. Maggie has worked as a photographer, a writer, and occasionally modeled. She also noted Montana's high suicide rate and the empathy she feels for the people there going through the same thing.
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