Michael J. Fox, whose turn from Parkinson's disease patient to scientific crusader made him one of the country's most visible advocates for stem cell research, now believes the controversial therapy may not ultimately yield a cure for his disease, he told ABC's Diane Sawyer in an exclusive interview.

There have been "problems along the way," Fox said of stem cell studies, for which he has long advocated.   Instead, he said, new drug therapies are showing real promise and are "closer today" to providing a cure for Parkinson's disease, a degenerative illness that over time causes the body to become rigid and the brain to shut down.

"Stem cells are an avenue of research that we've pursued and continue to pursue but it's part of a broad portfolio of things that we look at. There have been some issues with stem cells, some problems along the way," said Fox, who suffers from the diseases' telltale tics and tremors.

"It's not so much that [stem cell research has] diminished in its prospects for breakthroughs as much as it's the other avenues of research have grown and multiplied and become as much or more promising. So, an answer may come from stem cell research but it's more than likely to come from another area," he said.

Tune in to "World News with Diane Sawyer" Friday at 6:30 p.m. E.T. to see more of Diane Sawyer's interview with Michael J. Fox 

Fox, who recently appeared in episodes of "Curb Your Enthusiasm" and "The Good Wife," has dedicated himself to finding a cure for Parkinson's, the disease with which he was diagnosed in 1991.

Fox said he still strongly believes in stem cell research and government support of those studies, praising ongoing research at New York's Memorial Sloan-Kettering Hospital. When asked about earlier criticism he received from conservative talk show host Rush Limbaugh about his advocacy, Fox said it only "sharpens your resolve."

Scientists are conducting research and looking for a cure on multiple fronts, Fox said, including drug therapies, experimental surgeries, and developing tests to help make earlier diagnoses.

To that end, his Michael J. Fox Foundation for Parkinson's Research, the largest private funder of Parkinson's disease research worldwide,  has recently launched an online initiative to increase studies across the country by pairing patients with clinical trials in their areas.

The Fox Trial Finder (Visit FoxTrialFinder.org for more info on clinical trial participation) harnesses the power of the Internet to find patients and, based on their profile of symptoms, pair them with research scientists conducting clinical trials.

Thirty percent of all clinical trials fail to recruit a single subject,  according to the foundation's web site, and many more, some 85 percent, are delayed because scientists are unable to find enough participants.

"People can fill out a form anonymously… and then we can let them know about… clinical trials happening in their area," Fox said.

Some 200 trials are currently seeking recruits through the website, but one of the most promising will "try to find a biomarker for Parkinson's, which is really important," Fox said.

By the time Fox was diagnosed 20 years ago, he said, 80 percent of the dopamine cells in his brain - neurons instrumental in sending the signals that control movement - were depleted.

"We have no way to identify the disease before symptoms appear. If we can target progress along the way, we can arrest progress and eliminate the possibility of symptoms," he said, adding that this area of research is "the most exciting."

Fox, an actor who grew up in front of the camera on the 1980s sitcom "Family Ties" and starred in the "Back to the Future" film franchise, worried about the future of his career after announcing his diagnosis in 1998 and leaving the hit show "Spin City."

In the years since, he has led the Fox Foundation, which has donated more than $300 million to Parkinson's research.  In recent months, however, he has returned to acting more regularly, the result he says, of a new drug regimen that helps control his tics, or dyskinesia.

"I kind of stumbled onto a new combination of meds for what's called dyskinesia… Now I thought, there's no reason not to work so I started to accept more work. Larry David called and had a terrific idea and the 'Good Wife' is such a terrific show," he said of his decisions to appear on HBO's "Curb Your Enthusiasm" and the CBS series "The Good Wife."

Fox said that each morning he is uncertain exactly how his symptoms will affect him that day. Some mornings he can delay taking his first dose of medicine for a few hours, other days he expects a greater challenge.

"I don't write off the day ahead of time because of that, it just means it's going be tougher sledding," he said.

Having struggled with the disease for years himself, Fox understands its devastating effects and the physical challenges it presents.

He said it was an abiding sense of optimism, a topic on which he has written two books, that allows him to carry on, even on the most difficult days. In 2009, he traveled to the Asian country Bhutan, which emphasizes happiness over productivity, and said he found his symptoms diminished there.

"People talk about me being a paragon of optimism and hope and all that stuff," he said.  "I have a really blessed life, I have an amazing life."