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:


Poison-control center? For ingesting toothpaste? ''I mean, what does that do to you?'' a worried Ms. 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.''

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 that 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 F.D.A., guardian of the nation's food supply, of going overboard in this case. What particularly galls them is that the crackdown is regarding 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 led to 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 vomited -- 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,'' Mr. Vanegas added. ''This sounds like you should get an ambulance over immediately.''

No ambulances will be necessary. The F.D.A. ordained the advisory not because some new study suggested more serious side effects, but because it believes that 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. The main threat of swallowing too much fluoride, the association says, is a condition called fluorosis, which causes white spots on the teeth. The old toothpaste warnings, written by the dental association, were directed at that problem.

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

''There always was a warning,'' said Dr. Linda Katz, deputy director for over-the-counter drug products at the F.D.A., ''but I don't think it really hit the message that fluoride is a drug and has been associated with both acute and chronic toxicity.''

Nobody disputes the need for the agency to keep close tabs on the ingredients of things that end up in people's mouths. But fluoride is hardly a threat to life and limb. Except in rare cases of allergic reaction resulting in continued vomiting and deyhydration, the worst symptoms of fluoride poisoning in children are diarrhea and nausea. Of the 4,453 cases of unintended ''fluoride exposure'' reported to poison-control centers last year, 99 percent were minor and not one was life-threatening.

Fluoride causes vomiting because in large doses and in combination with gastric juices, it can irritate the stomach and intestines. Opponents of the fluoridation of drinking water fear longer-term health consequences, but the F.D.A. warning was not designed to address those concerns.

Yet the new wording might as well be on a container of bleach, toothpaste manufacturers and dentists say.

''The industry believed that this label was inappropriate for toothpaste and would not be required because it was absolutely diametrically opposed to the public health policy of encouraging the use of fluoride for the reduction of dental cavities,'' said David Richardson, vice president of regulatory affairs and product safety for the Colgate-Palmolive Company in New York.

The rewording was the result of a comprehensive scientific review of over-the-counter drugs initiated by the F.D.A. in the 1970's to insure that their ingredients were safe and their labeling informative. Since then, advisory panels made up of doctors, scientists and consumer advocates have been inspecting one product after another.

The agency's order could not have come at a worse time for the $1.5 billion toothpaste industry. Manufacturers are engaged in a fierce marketing battle that has Colgate-Palmolive putting $100 million behind its newest toothpaste, Colgate Total, while the Procter & Gamble Company is spending $40 million on its new Crest Extra Whitening.

But the pesky warning is undercutting their pitches somewhat and forcing them to take steps to reassure the public.

Unilever PLC, the maker of Mentadent, has put information on its Web site. Procter & Gamble has been including a separate message inside some packages of Crest Multicare advising customers to stop using it if they experience irritation. Procter & Gamble declined to comment for this article, but its toll-free customer-service number provides an upbeat recorded message.

Actually, the F.D.A.'s own package-size limitations have long been designed to prevent fluoride poisoning. Even though some brands, like Oral B bubble-gum flavor and the Tom's of Maine ''Silly Strawberry'' formula, are made to be appetizing to small children, no child could get through an entire family-sized six-ounce tube without vomiting, the ideal treatment for an overdose.