Q. What happens if you use toothpaste that’s past its expiration date? Is it dangerous?

A. No, it’s not dangerous, said Dr. Joel H. Berg, chairman of pediatric dentistry at the University of Washington in Seattle and a representative of the American Academy of Pediatric Dentistry.

“But,” he said, “there is a potential loss of efficacy. It depends on how long past the expiration date it is.”

All toothpastes containing fluoride are regulated by the Food and Drug Administration, which now requires expiration dates, said Veronica Sanchez, a spokeswoman for Procter & Gamble, which makes Crest, the first fluoride toothpaste.

Up to that date, normally two years after manufacture, all the fluoride must be available to bind to tooth enamel, hardening it against the acid that causes cavities.

Once that date is past — and probably long past, Dr. Berg added — some of the fluoride ions may have bound with the caking agents into a salt or a crystal.

And, he said, depending how long and at what temperatures the tube was stored, the goo inside could separate, meaning less or more fluoride in each squeeze, and less or more flavoring agent, which could be mintily disconcerting.

Also, he added, “you might see viscosity changes, meaning you won’t be able to get it out of the tube, or it won’t foam in the mouth very well.”

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