Colgate-Palmolive introduced that product in August, and it may be the only popular mouthwash to contain MI, an ingredient added to deter the growth of bacteria. But it can cause rashes and skin irritations in people who are allergic to it. The company chose that preservative, according to Stephanie Clark, a spokeswoman for Colgate, because it worked best for that particular formula.

Experts say it’s an unusual additive for a mouthwash, an expansion of the chemical into products occurring at the same time that major consumer products companies — including Kimberly-Clark, Johnson & Johnson and Unilever — have begun removing it from lotions and wipes.

“It’s not a preservative that we’ve seen commonly in products that are used anywhere but on the skin,” Dr. Bruce A. Brod, president-elect of the American Contact Dermatitis Society, said. “When people are exposed on a daily basis to MI, we think the rates of sensitization are going up.”

Compounding the problem, Dr. Brod said, is a lack of awareness among both patients and doctors. Ms. Kirn said her hands “looked like I had been in a fire” several months after she began using Huggies Simply Clean wipes, but she didn’t discover her allergy to the product until months later.

The dermatitis society named MI its “allergen of the year” in 2013, a listing intended to give attention to problematic and often obscure substances. That same year, the Scientific Committee on Consumer Safety, a European advisory group, said that MI should be used only in limited quantities for rinse-off products, like soaps and shampoos, and that “no safe concentrations” existed for leave-on products like lotions.