Michael J Fox admits his Parkinson’s has become “harder over the years” but he’s promised to find a cure.

The 65-year-old actor, best known for his roles in the likes of Back To The Future, Spin City and most recently Shrinking, has raised more than $US3 billion ($A4.2 billion) in research for the disease since founding the Michael J Foundation in 2000.

Reflecting on balancing the spotlight with handling his own symptoms, he told PEOPLE magazine: “Of course, things have gotten harder over the years.

“But I’m privileged to commit my time and energy to bringing everything I’ve got to help cure this disease.”

Fox effectively became a public face of the disease when he launched the foundation, and he pointed out that his own experiences help.

He said: “I know the issues and I share my community’s sense of urgency.”

The beloved actor first retired in 2000 after leaving Spin City when he went public with his Parkinson’s diagnosis.

He recently joined the cast of Shrinking, playing someone with Parkinson’s opposite Harrison Ford, whose character Dr. Paul Rhoades also has the disease.

He decided to step back into the acting world in 2020 after “a new variable emerged”, which was his “ability to memorise lines”.

Fox said: “There’s a time for everything, and the time for 12-hour workdays was behind me.”

He is open to returning to Shrinking for the show’s fourth season, but his focus is on his foundation.

He explained: “The Foundation’s work is about driving toward a world without Parkinson’s. We don’t know exactly when it’ll happen, but it will happen.”

The foundation is hoping to raise another $US2.5 billion ($A3.5 billion) for research with a new campaign, and the star noted that “science is hard” and “astronomically expensive”.

He said: “It takes time and collaboration. But as long as we stay focused, we stay hopeful, we’ll get there. And the science is showing we’re getting close.”

Fox previously explained how he tries to keep his sense of humour “intact” amid his battle with Parkinson’s, which causes uncontrollable movements, such as shaking, stiffness, and difficulty with balance and coordination.

He told PEOPLE: “It’s hard for me. But I gotta keep it intact.”