Menthol needs no introduction: it's the thing that causes the cooling sensation so well-known both from cosmetic products as well as a bunch of other things like chocolate, chewing gum, toothpaste or cigarette. It's a natural compound that comes from the essential oil of Mentha species (peppermint oil contains 40-50% menthol) and it gives them their typical minty smell and flavor.
As for skincare, menthol seems to be a mixed bag. Apart from the cool cooling sensation (that might last up to 70 mins!), it also has painkilling, itch reducing, antibacterial, antifungal and even penetration enhancing properties. On the other hand, it also seems to act as a skin irritant that increases trans-epidermal water loss (the water that evaporates from the outer layer of the skin) and thus contributes to drying out the skin.
Show me some proof
- Phytochemistry 96 (2013) 15–25, Menthol: A simple monoterpene with remarkable biological properties
- Archives for Dermatological Research 288(5-6):245-8 · May 199, Effect of topically applied menthol on thermal, pain and itch sensations and biophysical properties of the skin