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 hundreds of 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.

Agency officials generally follow the recommendations of the advisory panels, though not always, and it could take some months before a final decision is made. And the findings could be contested, prolonging any move toward product substitutions or removing certain stock at stores.

In the meantime, experts advised consumers not to panic or toss out all the drugs in their medicine cabinet. Even though the agency’s advisers have decided the ingredient, phenylephrine, doesn’t work to relieve nasal congestion when taken orally, it is not dangerous, and the products do contain other ingredients that will work to ease cold symptoms.

The panel’s vote followed its review on Monday and Tuesday of several existing studies, with the advisers largely concluding 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.