On "Rescue Me," his character doesn't hesitate to jump into blazing buildings or self-destructive situations. In real life, Dennis Leary's jumping into the ring with Amanda Peet and Jenny McCarthy in the battle about autism.
In his new book, "Why We Suck: A Feel-Good Guide to Staying Fat, Loud, Lazy and Stupid," Leary, who has two sons, lashes out against parents who think their kids are autistic.
"There is a huge boom in autism right now because inattentive mothers and competitive dads want an explanation for why their dumb-ass kids can't compete academically, so they throw money into the happy laps of shrinks ... to get back diagnoses that help explain away the deficiencies of their junior morons. I don't give an [expletive] what these crackerjack whack jobs tell you -- yer kid is NOT autistic. He's just stupid. Or lazy. Or both," he writes.
Leary's not the first celebrity to weigh in on a debate that may be best handled by the medical community, not Hollywood. In August, Amanda Peet spoke out about autism and childhood vaccination on ABC's "Good Morning America," saying the public might be better off to turn a deaf ear to celebrities when it comes to vaccines.
"It seems that the media is often giving celebrities and actors more authority on this issue than they are giving the experts," Peet said. "I know it's a paradox, but that's part of why I wanted to become a spokesperson, to say to people, 'Please don't listen to me. Don't listen to actors. Go to the experts.'"
Peet also apologized again during her appearance for comments she made in the July issue of the parenting magazine Cookie, in which she stated, "Frankly, I feel that parents who don't vaccinate their children are parasites."
"I didn't mean to show disdain, and I did and do apologize for the use of the word 'parasites,'" she said. "But I do in no way, shape or form retract my position or the meaning behind the use of the word, which is that if there are vast reductions in herd immunity, our children will be at risk."
Peet has taken her message to the Internet, sharing her story on www.vaccinateyourbaby.org. And the actress, who stars in the new movie "X-Files: I Want to Believe," last month announced her support of the group Every Child By Two (ECBT). She is scheduled to be part of a panel on Aug. 5 to encourage parents to have their children receive all recommended vaccinations by the age of 2.
Peet says her interest in vaccines began with the birth of her daughter, Frankie, on Feb. 23, 2007.
"When Frankie was born, I started to learn about vaccine safety," she told ABCNews.com in an e-mail message. "The more I learned, the more I realized how much misinformation there is about vaccines."
Amy Pisani, executive director of ECBT, says she hopes Peet's advocacy is "a 100 percent antidote" to former Playboy model Jenny McCarthy's position that the current vaccine schedule places children at a higher risk of developing autism.
Pisani says that the issue is too important to be reduced to a showdown between celebrities.
"We don't want it to be a fight between Jenny McCarthy and Amanda Peet," she says. "This is between scientists and the public."
Still, the entrée of the latest celebrity voice into the vaccine debate has made waves online. In a message on ECBT's Web site, Peet says she made the decision to support the organization when she was pregnant.
"Many of our friends in Hollywood were choosing not to vaccinate their babies or to delay vaccines because they feared that they might cause autism or other disorders," she notes in her message. "We had never thought about not vaccinating, but when we went online to educate ourselves on vaccines, we found Web site after Web site warning us about their dangers. Naturally, we were very concerned about what we read."
Peet says her discussions with doctors at once allayed her fears and introduced her to the dangers of skipping vaccinations. It is this platform that she has pushed in recent weeks -- and some of her comments have been a source of ire for those who believe there is a link between childhood vaccinations and autism.
In particular, Peet's "parasite" comment has been a sore spot for many parents who believe in a vaccine-autism link.
Rebecca Estepp, parent support and media relations manager for the advocacy group Talk About Curing Autism (TACA) -- of which McCarthy is a spokeswoman -- says the star's comments were out of line.
"I guess she's calling me a parasite, because I did not vaccinate my second son because of what I saw happened to my first son," says Estepp, who adds that her first child developed autism shortly after his vaccination.
Science as a Celebrity Cause?
While celebrity advocacy of certain health causes is not a new phenomenon, having McCarthy and Peet squaring off over the vaccine debate has taken public interest in the issue to another level.
Dr. Paul Offit, chief of the section of infectious diseases at the Children's Hospital of Philadelphia, says much of this has to do with the fact that the public, as a whole is, in many ways, more eager to listen to celebrities than to researchers.
"I think when Jenny McCarthy appears on "Oprah" and says, 'I think vaccines harm our kids,' that has an effect. When Amanda Peet comes out and says, 'I believe the scientists on this,' that also has an effect," Offit says. "It's a culture of celebrity, and we tend to trust what celebrities tell us."
Offit, a leading advocate of the current vaccine schedule, is the doctor whose advice Peet sought early in her search for information. Peet met with Offit through her brother-in-law, who is a fellow in the Children's Hospital of Philadelphia's division of infectious diseases. And Offit says that what Peet learned about the risks of skipping vaccinations is what drove her to take action.
"It angered her," he says. "She felt like this, at some level, put her children at risk."
As for McCarthy, Estepp says the former model approached TACA in 2005 after her own son had been diagnosed with autism.
"When [her son] got better, she said, 'I want to help,'" Estepp says, adding that McCarthy's advocacy grabbed the attention of other celebrities. "It looks like Jenny's story kind of sparked like wildfire through the Hollywood community."
Offit, too, notes that Hollywood has become a major battleground in the vaccination debate. And he says that while scientific evidence on the importance of vaccination is irrefutable, celebrity culture goes a long way in shaping public discourse on the topic.
"[Peet] lives, to some extent, in the California culture and knows that people like Jenny McCarthy are not the minority there," he says.
But Peet says she bases her arguments for vaccination on the expertise of doctors and medical groups.
"I don't think parents should be taking medical advice from actors," she told ABCNews.com via email. "I take medical advice from several pediatricians, other doctors, the CDC, and the American Academy of Pediatrics."
Following Peet's appearance on "Good Morning America," the American Academy of Pediatrics issued a statement further clarifying its position on the matter.
"We know vaccines protect children against potentially deadly and disabling diseases," the statement reads. "Vaccine side effects are usually minor and include fever, mild swelling and pain at the injection site. In unusual cases, such as when children are ill or have certain chronic conditions, a pediatrician will advise delaying or skipping certain vaccines.
"Many valid scientific studies have shown there is no link between vaccines and autism," the statement continues. "The American Academy of Pediatrics supports further research into the causes of autism in hopes this will lead to optimal strategies for prevention, diagnosis and treatment."
Offit says Peet's approach sets her apart from other celebrities weighing in on the topic.
"[Peet] says, 'I'm not a scientist; you shouldn't even be listening to me,' which is refreshing," he says.
Beyond Celebrity, Debate Rages
While the participation of Peet and McCarthy in the vaccination debate may make waves among the public, their convictions are unlikely to sway those of medical experts and autism advocacy groups.
"The risks [of skipping vaccinations] became no longer theoretical with the recent measles outbreaks, which have sickened 127 children in 15 states," Offit says. "Measles makes you sick. One out of every 1,000 kids who gets it dies from it, and one in five are hospitalized."
"We do not endorse vaccination for all children, and with the present schedule, because of the lack of safety testing and the number of reports from parents about losing their child after vaccination," counters Stan Kurtz, executive director of the advocacy group Generation Rescue, in an e-mail to ABCNews.com. "My child and Jenny McCarthy's child included."
But Offit says, no matter what science may say, there is little doubt that celebrity advocacy goes a long way in swaying public opinion -- and public health.
"We're in an information age more than any other time in history, and I think that's both good and bad," he says. "It's good because we have a lot of information to work with, but it's bad because some people think that's enough to make them experts. That's not true here."
This article was updated on Aug. 15 to include a statement from the American Academy of Pediatrics.