Nobody can be sure, though. The science is not far enough along, partly because our regulation of toxins is so limp. Companies don’t have to release much of their internal safety data. And regulators face a terribly high burden of proof. They can often take action only after they have demonstrated that a substance is harmful — a task that corporate secrecy can make impossible.

“I can get information on only 20 percent of chemicals we interact with on a daily basis,” says Alan Goldberg, a toxicologist at Johns Hopkins. Erik Olson, a food and consumer product expert at the Pew Charitable Trusts, sums up the situation this way: “We’re a heck of a lot closer to the Wild West than the nanny state.”

The story of denture cream and zinc is a good example. A dentist in the Navy noticed the link between zinc and copper deficiency in the 1950s, according to Dr. Harold Sandstead of the University of Texas in Galveston. Studies in later years confirmed the relationship. Early last decade, researchers made the connection from excess zinc to copper deficiency to neurological problems. “It’s nothing new,” Peter Hedera, a Vanderbilt University neurologist, told me. “If you researched the field, you would find out.”

In 2008, Dr. Sharon Nations of Southwestern Medical Center in Dallas and other researchers published a study in the journal Neurology that took the research one step further. It specifically tied denture cream to severe neuropathy. Dr. Hedera followed up that study with another one analyzing 11 patients with high, unexplained zinc levels. To his surprise, all 11 turned out to be heavy users of denture cream.

Yet even after those studies appeared, GlaxoSmithKline continued to sell Poligrip. The company simply inserted a small piece of paper into the product’s box containing some mild statements that barely even seemed to be warnings. The headline on the insert was, “For Best Results Start With a Small Amount.”

Perhaps even more questionable than GlaxoSmithKline’s response has been that of Procter & Gamble, the giant consumer products company that also makes Crest, Tide, Pampers and Head & Shoulders. Procter is still selling a denture cream with zinc in it. Why? The cream, Fixodent, has only about half as much zinc as Poligrip did.

Even so, it may be enough to cause problems. Some of the 11 patients in the Hedera study were Fixodent users. “I would withdraw both” — not just Poligrip but also Fixodent, Dr. Hedera says.