Mark Jacoby had no idea why his body was failing. Symptoms appeared gradually, said the 41-year-old former construction worker from York, Pa.
"I started getting tingling in my fingertips. And then it started happening in my toes," he told ABC News' 20/20 anchor Chris Cuomo, who is the Chief of the Law & Justice Unit. "I started getting weaker and, you know, I couldn't walk right, off balance and I'm at this point now."
Jacoby, who now uses a wheelchair, said his doctors searched for years for the cause of his debilitating neurological illness that robbed him of his independence.
Doctors eventually tied his disorder to high levels of the mineral zinc in his body. Jacoby, who has worn dentures for 20 years, said it came from his denture cream, Fixodent, which contains zinc.
"I can almost guarantee you it was the Fixodent," said Jacoby. "It's soaked into your body and it messes with the nerves."
Similarly, 48-year-old Anne Coffman from Maine said she was a heavy user of Fixodent because of ill-fitting dentures, and after three years her body felt different.
"I started getting numbness in my toes. I wasn't sure so I kind of didn't do anything about it at first," Coffman told Cuomo. "Then it started moving over to both feet and then ... up to my knees."
After many painful tests, she was diagnosed with zinc poisoning, a condition in which high zinc levels interfere with the body's absorption of copper, which can lead to serious and irreversible neurological problems.
"[Fixodent] is the only product that I've ever used that had zinc in it," Coffman said.
Coffman is now in a wheelchair and struggles with everyday tasks because of weakness in her hands and arms.
"I have trouble eating," said Coffman. "I drop the fork a lot, so it gets frustrating."
Coffman and Jacoby are part of a class action lawsuit against Procter & Gamble, makers of Fixodent, alleging that their use of the product has caused their devastating problems.
"I don't know if you can put a dollar value on my health or anybody else's, for that matter," Coffman told Cuomo. "I would prefer to see [the makers] take the zinc right out of the product ... or take the product right off the market."
Used by many of the 35 million Americans who wear dentures, the cream is marketed with the catchy tag line, "Fixodent -- and forget it."
But what the maker didn't tell consumers for years was that the adhesive contains zinc, which when ingested or absorbed in large amounts over time can lead to serious nerve damage.
The possible connection first was made five years ago by researchers at the University of Texas, who studied four denture users with neurological disease.
"They had high zinc levels that we could measure in the blood," said Dr. Sharon Nations, author of a study in the journal Neurology. "And all of them reported that they were using very large amounts of denture cream."
That study was completed in 2006, but its publication in Neurology was delayed for more than two years.
It was delayed, according to its authors, because of a peer review by Dr. Kenneth Shay, a dentist, who lambasted the study and called the link between excessive use of denture cream and neurological disease "little more than speculation." He said that the authors "don't understand the nature of the material they are writing about."
But an ABC News investigation found that that Shay, at the time, also was a paid consultant to Procter & Gamble, the maker of Fixodent, when he reviewed the study.
"It is an outrage. This was wrong," David Rothman, a professor at Columbia University Medical School, told Cuomo. "That is a fundamental transgression of professional medical ethics and not to be allowed."
In e-mails and documents obtained by ABC News, Shay not only made recommendations that, according to the authors, led them to water down the study's finding, but also sent draft reports of the study to Procter & Gamble.
In one e-mail, he said, "Please be circumspect because, as a reviewer, I'm not supposed to be passing an unpublished manuscript around."
It is unclear what, if anything, Procter & Gamble did with the information Shay passed on to it.
It wasn't until 2009, after the study was published in the journal Neurology, that Procter & Gamble added a "new label information" warning to the side of Fixodent packages, and on the back, cautioned that "prolonged zinc intake may be linked to adverse health effects."
Shay declined to be interviewed, but in a phone call with ABC News he defended his review of the study, saying that the research report had "objective shortcomings."
Procter & Gamble sent a written statement to ABC News reading:
"Procter & Gamble is committed to providing safe and effective products for all consumers. That is who we are and what we stand for. We go to great lengths to ensure that our products safely deliver best-in-class performance, so consumers can choose our products with total confidence. Fixodent is safe for use as directed, as supported by the experience of millions of consumers over many years. Our Fixodent formula has undergone extensive scientific testing, and we continuously monitor for its safe use. ... We know of no valid scientific evidence that using Fixodent as directed causes any ill health effects."
But Mark Jacoby and others like him said it all comes too late. He has stopped using Fixodent, but said the damage to his body has been done.
"Fixodent and forget it?" said Jacoby. "Well, apparently I can't forget it 'cause it took a lot away from me."
If you have suffered from adverse reaction to Fixodent, send your story to Chris Cuomo.