Menthol
In plain terms
The compound that makes mint feel cold and tingly. It switches on the skin and mouth's cold-sensing nerves without any real drop in temperature, which is why it turns up in lip balm, muscle rubs, toothpaste, and cough drops.
What the evidence shows
Menthol is the cooling agent in mint, and for most people on adult skin it is well tolerated. The FDA treats it as generally recognized as safe, the National Toxicology Program found it neither genotoxic nor carcinogenic, and the Cosmetic Ingredient Review panel judged peppermint-derived menthol safe at the levels products use. Two limits sit alongside that. It is a recognized contact allergen and irritant for a minority, so mint-flavored toothpaste, lip products, and topical rubs can bring on burning, redness, or an allergic rash. The sharper caution is age. Mentholated rubs and drops are not for babies and young children, because menthol and its usual partner camphor are readily absorbed and can slow or interrupt an infant's breathing, and in a newborn with the inherited condition G6PD deficiency they can trigger red blood cells to break apart. So the picture turns on who is using it and where: a cooling adult lip balm or muscle rub is a low concern, while products for infants, and use on broken or sensitive skin, are where the caution belongs.
On the label
Listed as menthol or levomenthol, usually well down the ingredient list. On products for babies and toddlers, a menthol rub or vapor product is the thing to avoid; for adults it is mainly a flag if mint has irritated your skin or lips before.