The death of Margot Kidder, an actor best known for her role in Superman, has been ruled a suicide.

Kidder, who played Lois Lane opposite Christopher Reeve’s Superman, was found by a friend in her Montana home on 13 May. At the time Kidder’s manager, Camilla Fluxman Pines, said she had died peacefully in her sleep.

Her daughter Maggie McGuane said on Wednesday that it was a “big relief” to have the truth out. “It’s important to be open and honest so there’s not a cloud of shame in dealing with this.”

A statement released on Wednesday by the Park county coroner Richard Wood said the star died from a self-inflicted drug and alcohol overdose, and no further details would be released.

McGuane urged people with mental illness to seek help. “It’s a very unique sort of grief and pain,” McGuane said. “Knowing how many families in this state go through this, I wish that I could reach out to each one of them.”

Kidder dealt with mental illness for much of her life, while a 1990 car accident left her in debt and needing a wheelchair for almost two years.

Kidder and Reeve starred in four Superman movies between 1978 and 1987. She also appeared in The Great Waldo Pepper with Robert Redford in 1975, Brian De Palma’s Sisters in 1973 and The Amityville Horror in 1979.

She later appeared in small films and television shows until 2017, including RL Stine’s The Haunting Hour. She received a Daytime Emmy Award as outstanding performer in a children’s series in 2015 for that role.

Kidder, originally from Yellowknife, Canada, was a political activist who was arrested in 2011 in a Washington DC protest over the proposed Keystone XL pipeline from Canada’s oil sands.

Joan Kesich, a longtime friend who found Kidder’s body, said she had been fearless and always spoke the truth, regardless of the consequences.

“In her last months, she was herself – same kind of love, same kind of energy,” Kesich said. “The challenges that she had were very public. I want what I know about her to be out there because it was glorious. She was really a blazing energy.”

• In the UK the Samaritans can be contacted on 116 123. In the US the National Suicide Prevention Lifeline is 1-800-273-8255. In Australia the crisis support service Lifeline is 13 11 14. Other international suicide helplines can be found at www.befrienders.org