Updated

In a response to charges by conservative talk-show host Rush Limbaugh, Michael J. Fox defended his appearance in recent political campaign ads, saying he was neither acting nor off his medication for Parkinson's disease.

On the contrary, he had been overmedicated, the actor said during an interview aired on Thursday's "CBS Evening News with Katie Couric."

"The irony of it is that I was too medicated," Fox told Couric, adding that his jumpy condition as he spoke to her reflected "a dearth of medication — not by design. I just take it, and it kicks in when it kicks in."

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"That's funny — the notion that you could calculate it for effect," he said. "Would that we could."

The 7 1/2-minute interview with Fox, whose shaking at one point dislodged the microphone clipped to his jacket lapel, aired in two segments taped Thursday afternoon on the "Evening News" set.

Fox drew some conservative criticism after an ad began running in Missouri during the World Series. It showed Fox visibly shaking while urging fans to vote yes for stem-cell research and Democratic Senate challenger Claire McCaskill — and no to the Republican incumbent Jim Talent.

"They say all politics is local, but it's not always the case," Fox says in the 30-second spot. "What you do in Missouri matters to millions of Americans — Americans like me."

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Fox, who supports research on embryonic stem cell for a potential cure for Parkinson's, also has lent his celebrity to Democrats Rep. Benjamin L. Cardin, running for the Senate in Maryland, and Wisconsin Gov. Jim Doyle, who is seeking re-election. Both politicians also back stem-cell research.

The ads triggered a backlash, with radio commentator Limbaugh claiming during his broadcast that Fox was "either off his medication or acting." Limbaugh called Fox "really shameless" in his effort to stir up sympathy. Limbaugh apologized later in the broadcast.

Fox told Couric that he did the ads only to advance his cause, and that he doesn't care about politics.

"Disease is a nonpartisan problem that requires a bipartisan solution," he said.

"Would you support a Republican candidate?" Couric asked.

"I have," Fox replied. "I've campaigned for Arlen Specter," describing the Republican Senator from Pennsylvania as a "fantastic champion of stem-cell research."

"We have a right if there's answers out there, to pursue those answers with the full support of our politicians," he said.

Fox, 45, who starred on TV's "Family Ties" and "Spin City" as well as the "Back to the Future" films, was diagnosed with Parkinson's in 1991 and revealed his condition publicly in 1998. In 2000, he quit full-time acting because of his symptoms and founded the Michael J. Fox Foundation for Parkinson's Research, which has raised millions of dollars.

At the conclusion of the first segment of the interview, Couric told viewers that her father has Parkinson's and that she has made contributions for research to Fox's foundation.

For complete election coverage, go to FOXNews.com's You Decide 2006 center.