The coroner in Park County, Mont. said on Wednesday that Kidder "died as a result of a self-inflicted drug and alcohol overdose".
Kidder, who played Lois Lane opposite Christopher Reeve's Superman in her most famous role, was found by a friend in her Montana home on May 13.
Kidder's daughter, Maggie McGuane - during a chat with the Associated Press - said she and the rest of their family wanted to be honest about her death to avoid the "cloud of shame". But now her daughter, Maggie McGuane, has revealed that her mom committed suicide.
McGuane went on to tell the AP that Montana has "one of the highest suicide rates in the nation" and urges those with mental illness to seek help.
Britons split over Boris Johnson’s burka comments
Metropolitan Police Commissioner Cressida Dick said Mr Johnson's comments would not "reach the bar" for a criminal offence. Schools and universities should be able to take the same approach if a student "turns up. looking like a bank robber".
She continued to act in her later years but her popularity waned after she suffered a highly publicised mental breakdown in 1996. Her film "Robber's Roost" was in pre-production, and the final film she worked on was 2017's "The Neighborhood".
In what was an obvious reference to the Superman movies, Kidder made a cameo in Smallville along with the series' stars Michael Rosenbaum and Allison Mack.
Canadian-born Kidder also appeared in 1975's The Great Waldo Pepper with Robert Redford and 1978's The Amityville Horror.
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. The coroner did not specify which drugs she took, TMZ reported.
Thousands evacuated after Lombok quake kills 98
Lombok airport's general manager said airlines had laid on extra flights and his staff had been providing blankets and snacks. A mother bath her daughter in a bucket after their home in Sembalun Bumbung was destroyed by the Lombok quake .
Kidder and other environmentalists protest outside the White House in Washington in August 2011.
A serious auto accident in 1990 left Kidder temporarily unable to work, putting her in debt.
"In her last months, she was herself - same kind of love, same kind of energy", Kesich said.
"The reality of my life has been grand and wonderful, punctuated by these odd blips and burps of madness", she said.
New York City has hit Uber with a cap on new cars
Drivers previously pushed for a cap on new competition in 2015, but were beaten back by ride-hailing companies. The legislation also allows the city to set a minimum pay rate for drivers and minimum fares for the industry.