NEW YORK — It was a routine purchase of that most benign of products, toothpaste, at a Duane Reade drugstore in Manhattan. So Laura Cheek, mother of a 22-month-old daughter, did a double take when she read the new message on the tube of Colgate that she had just pulled off the shelf:

“Warning: Keep out of the reach of children under 6 years of age. If you accidentally swallow more than used for brushing, seek professional assistance or contact a Poison Control Center immediately.”

Poison-control center? For ingesting toothpaste? “I mean what does that do to you?” Cheek asked, musing out loud whether she should continue helping her toddler to brush. “I don’t want to use it if it will harm you,” she said.

Well, it won’t. But the Food and Drug Administration wants to be on the safe side — too safe, some say. The agency required the new warning on all fluoride pastes manufactured after April 1997 after concluding manufacturers were either ignoring its voluntary guidelines or interpreting them too broadly. The change is only now catching most consumers’ attention as stores sell out old inventories.

But critics, including the American Dental Association, toothpaste manufacturers and some consumers, accuse the FDA — guardian of the nation’s food supply and scourge of Big Tobacco — of going overboard in this case. What particularly galls them is that the crackdown targets a chemical that has been hailed for decades as the first line of defense against tooth decay. Fluoride permeates the nation’s water supply and is routinely added to mouthwashes, soft drinks and canned goods, all in the cause of sparing children the dentist’s drill.

Worries about the labeling have triggered 500 calls to Colgate-Palmolive’s toll-free line, while Church & Dwight, the maker of Arm & Hammer toothpaste, has received nearly 200 calls. Toothpaste-related inquiries have surged at poison control centers, where operators tell parents not to worry if the child has not thrown up — and not to worry if he has. They reassure callers that vomiting is a normal reaction to fluoride ingestion and counsel them to seek medical treatment for possible dehydration only if the symptom persists.

For some people, the new phrasing, which replaces calmer notices like “Do not swallow — use only a pea-sized amount for children under 6,” comes as a shock. “Wow! I’ll have to call my brothers,” exclaimed Oscar Vanegas of Queens, the father of 12-year-old twins and the uncle of six young nieces and nephews, as he read the label on a tube of Colgate Tartar Control toothpaste at the Duane Reade drugstore.

“It’s a little scary,” he added. “This sounds like you should get an ambulance over immediately.”

No ambulances will be necessary. The FDA ordained the advisory not because some new study suggested more serious side effects, but because it believes any product that contains a substance classified as a drug should be labeled with a recommendation to seek professional help in cases of excess ingestion.

Still, the American Dental Association worries that some parents will overreact. It says the main threat of swallowing too much fluoride is a condition called fluorosis, which causes white spots on the teeth. The old toothpaste warnings, written by the dental association, were aimed at that problem.

The FDA emphasized that it is not suggesting that children should not brush with fluoride; it just wants to increase consumer awareness of possible side effects.