AceShowbiz - "Superman" star Margot Kidder's death has been ruled an intentional drug and alcohol overdose.

The Canadian actress and activist, 69, passed away at her home in Montana on May 13, prompting a police investigation.

Now, according to Park County Coroner, Kidder's overdose was self-inflicted.

The star, who played Lois Lane in the Christopher Reeve Superman movies, struggled with bipolar disorder for years, and she hit the headlines for all the wrong reasons in the mid-1990s, when her mental health battle went public and she was discovered living homeless.

Margot got to grips with her condition and returned to acting, landing roles on TV shows like "Smallville", "Brothers & Sisters", and "The L Word". She went on to pick up an Emmy Award for her performance on the children's television series R.L. Stine's "The Haunting Hour" in 2015, and she also found success on Broadway in a 2002 production of "The Vagina Monologues".

Kidder was acting up until her death.

As an activist, she became an advocate for mental health sufferers and was arrested in Washington, D.C. in August, 2011 for protesting against TransCanada Corp's Keystone XL pipeline plans. The campaigners were hoping to convince then-U.S. President Barack Obama to quash the $7 billion project to move oil from Alberta to America's Gulf Coast.