Margot Kidder's cause of death has been ruled as suicide by drug overdose.

The actress - who was best known for playing Louis Lane in four 'Superman' movies during the 1970s and 80s alongside the late Christopher Reeve - passed away at the age of 69 in May this year, and it has now been revealed that she took her own life.

According to TMZ.com, the Park County Coroner's Office says Margot died after a self-inflicted drug overdose, although it has not been revealed which drugs she took.

Margot was found dead at her home in Livingston, Montana, on May 13.

The actress battled bipolar disorder - which causes a person to have extremely elevated mood swings - for many years and had been open to the public about her struggle, even becoming an advocate for mental heath.

In 1996, her battle with her disorder was so intense that she was briefly made homeless after a breakdown which led her to disappear for four days.

At the time, she was found in a backyard by a homeowner and was taken by the Los Angeles Police Department to Olive View Medical Center in a manic state, and was later placed into psychiatric care.

Margot had a lucrative film and television career which included roles in 'Law & Order: Special Victims Unit', and she even returned to the 'Superman' franchise when she appeared in two episodes of 'Smallville' Bridgette Crosby, alongside her co-star Christopher, who then passed away in 2004.

Margot was married three times, to American novelist Thomas McGuane, actor John Heard - whom she married in 1979 but split from six days later - and French film director Philippe de Broca from 1983 to 1984.

The star is survived by her 41-year-old daughter Maggie, whom she had with her first husband.