Some companies, Lengyel says, are even experimenting with drugs that remove zinc from areas such as the brain, where it may be causing damage, and redistribute it to other cells that are deficient.

"It sounds a little like science fiction, but it has some scientific support," Lengyel says.

Denture cream

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Zinc raised other alarms last year, when researchers began to notice weakness, balance and memory issues and other neurological trouble in some patients. Sleuthing revealed the only common link: All of the patients used large amounts of denture cream enhanced with zinc.

Denture cream is meant to be used in small amounts, says Jaya Trivedi, a neurologist at the University of Texas Southwest Medical Center in Dallas. But people with poorly fitting dentures may end up using much more of it to keep their dentures in place. Trivedi and colleagues published their analysis of four denture-wearing, zinc-overloaded patients in the journal Neurology in 2008.

A tube of denture cream should last a month or more, Trivedi says. The patients in her study, however, had been using up to two tubes a week of Poligrip or Fixodent creams for many months or even years.

Zinc concentrations ranged from 17 to 34 milligrams per gram of denture cream, testing showed. That means that some people were exposed to as much as 330 milligrams of zinc a day, Trivedi says, though it's still not clear how much of that zinc actually got into their bloodstream. In some cases, nerve damage was permanent. Packages of Super Poligrip now include inserts telling people to talk to their doctors if also taking zinc supplements and to use the products as directed.

"We don't want everyone out there using denture cream to get scared. That's not our message," Trivedi says. Instead, she says, denture wearers should see a dentist if their dentures fit poorly, if they use a lot of cream or if they start to experience neurological problems.

Link to cancer

The brain isn't the only region of concern with zinc. The prostate contains some of the highest levels of zinc in the human body, and some evidence suggests a link between zinc and prostate cancer.

Zinc also builds up in the back of the retina in people with macular degeneration, Lengyel found in a 2007 study published in the journal Experimental Eye Research. And that may be causing unexpected problems.

Zinc is essential for proper eye function, and in an attempt to defy macular degeneration -- the major cause of blindness in elderly people in the Western world -- doctors have often prescribed supplements that contain 80 milligrams of zinc. In a small percentage of these cases, studies show, the metal helps slow the progression of blindness by a year or two.

More recently, however, Lengyel has found that people who take these extra-large supplements for years are 50% more likely to end up in the hospital with urinary tract problems. In a group of more than 3,500 people, about 11% had urinary complications, compared with about 7.5% of people who took zinc-free supplements.

"I don't think we have seen the real nasty path or potential of long-term supplementation," Lengyel says. "I don't think that's surprising. Any chemical you take in large excess is going to cause problems, even the good ones."

Nasal sprays

Researchers recommend avoiding nasal sprays containing zinc, as well. An October study in the journal PLoS One found that the Zicam brand of homeopathic zinc-enriched nasal spray caused long-term damage to the sense of smell in mice and signs of nasal nerve damage in people.

When it comes to supplements, sucking on zinc lozenges as soon as you get a cold may help and probably won't hurt, experts say, as long as you don't suck on them all day every day for the entire flu season. A week should be fine.

Some promising research is also starting to suggest that a tiny bit of copper supplementation can help override the dangers of getting too much zinc. In the meantime, experts suggest staying well the old-fashioned way: Eat a healthy diet, and stop staying up so late.