The actress, who portrayed White House Press Secretary C.J. Cregg on the longrunning political drama, slipped back into character as she returned to the Briefing Room podium at the presidential property during a press conference, pretending to stand in for real-life White House spokesman Josh Earnest.

"Hi everyone, good afternoon," Janney greeted reporters, before joking, "Josh is out today, I believe it's a root canal," a reference to one of her character's storylines on the show.

Earnest entered the room minutes later and quipped that while she was standing at the podium, she "might as well use it".

The 56-year-old actress then turned serious as she revealed the real reason for her appearance - to "draw attention to the opioid epidemic" and discuss the government's current plan to combat the widespread problem.

She went on to note how White House officials would be honouring 10 men and women who have played key roles in their local communities in tackling the drug epidemic, a topic which is also addressed on her TV sitcom, Mom.

"This is a disease that can touch anybody, and all of us can help reduce drug abuse through evidence-based treatment, prevention and recovery," she told the media. "Research shows it works, and courageous Americans show it works every day."

However, Janney couldn't resist using her visit to the White House to take one question as her The West Wing character.

Asked who Martin Sheen's President Jed Bartlett would support for the 2016 Democratic Party nomination, she smiled, "I think you know the answer to that question."

Her former co-star, Bradley Whitford, who played her Deputy Chief of Staff on the drama series, recently claimed Bartlett would back frontrunner Hillary Clinton.