The new guidelines, issued by the National Institute of Allergy and Infectious Diseases on Thursday, recommend giving babies puréed food or finger food containing peanut powder or extract before they are 6 months old, and even earlier if a child is prone to allergies and doctors say it is safe to do so. One should never give a baby whole peanuts or peanut bits, experts say, because they can be a choking hazard.

If broadly implemented, the new guidelines have the potential to dramatically lower the number of children who develop one of the most common and lethal food allergies, said Dr. Anthony Fauci, the institute’s director, who called the new approach “game changing.”

Could the new guidelines mark the end of the peanut-butter-and-jelly sandwich bans so common in school lunchrooms? “If we can put this into practice over a period of several years, I would be surprised if we would not see a dramatic decrease in the incidence of peanut allergies,” Dr. Fauci said.