An advisory panel to the Food and Drug Administration agreed unanimously on Tuesday that a common decongestant ingredient used in many over-the-counter cold medicines is ineffective.

The panel’s vote tees up a likely decision by the agency on whether to essentially ban the ingredient, phenylephrine, which would result in pulling products containing it from store shelves.

If the F.D.A. ordered their removal, a trade group warned that numerous popular products — including Tylenol, Mucinex and Benadryl cold and flu remedies — might become unavailable as companies race to reformulate them.

On Monday and Tuesday, the panel reviewed several existing studies and largely agreed that the research settled the question that the ingredient was useless and no better than a placebo.

Several advisers noted that patients taking the drug were merely delaying their journey to a useful remedy.

“I think we clearly have better options in the over-the-counter space to help our patients, and the studies do not support that this is an effective drug,” said Maria Coyle, the chairwoman of the panel and an associate professor of pharmacy at Ohio State University.

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Every cold and flu season, millions of Americans reach for these products, some over decades. The decongestant is in at least 250 products that were worth nearly $1.8 billion in sales last year, according to an agency presentation. Among the products: Sudafed Sinus Congestion, Tylenol Cold & Flu Severe, NyQuil Severe Cold & Flu, Theraflu Severe Cold Relief, Mucinex Sinus Max and others.

The ingredient has long been considered safe and effective under an old, outdated agency standard, and the F.D.A. still says that it is safe.

Many remedies that do include phenylephrine also contain other, more effective medicines as well.

And medications that are considered effective for sinus and nasal congestion do still include nasal sprays with phenylephrine, like Afrin, or oral pseudoephedrine, such as Sudafed, or nasal steroids, such as Flonase.

Many popular cold and flu products that don’t specifically target congestion do not include the ingredient.

And it is still widely considered effective when it is used in surgery and to dilate the eyes. It is destroyed in the gut, though, scientists have concluded.

If the agency decides the decongestant should be eliminated from products, it could significantly disrupt the market for the makers of cold medicines if they do not have enough time to replace it in popular items.

What’s more: It could possibly renew widespread use of an alternative, pseudoephedrine, which was placed behind store counters or in locked cabinets because it was often used in illicit meth labs.

Dr. Hendeles, now an emeritus professor, said in an interview on Tuesday that he had been evaluating the ingredient since 1993.

“The bottom line is quality research has told the true story about phenylephrine,” he said.

The F.D.A. has formally now concluded that phenylephrine, when taken orally, is “not effective as a nasal decongestant.”

For consumers, the potential benefits of ending use of the ingredient, the agency suggested, would include avoiding unnecessary costs or delays in care by “taking a drug that has no benefit.”