Menthol is a waxy substance which can be obtained naturally from mint oils, such as peppermint or spearmint. It is solid at room temperature, though it starts to melt at slightly higher temperatures. Because of its properties, menthol is in great demand, so much of it is created synthetically.
Menthol stimulates thermoreceptors that react to cold temperatures, causing a feeling of coolness. It doesn't actually lower temperature; rather, it tricks you into feeling cool because of its stimulation of those particular receptors. That's why menthol is widely used in topical or inhaled decongestants and in lip balm, toothpaste, mouthwash and gum.
Menthol helps relieve pain. It's especially soothing for sore throats and burns, including sunburns. It also relieves muscle soreness, since it's readily absorbed into the skin. Menthol is used in products to treat itching. Research is being conducted to discover the efficacy of menthol in working alongside other medicines to treat chronic pain.
Menthol can kill multiple kinds of bacteria, including Staphylococcus epidermidis and Escherichia coli. This effect seems to be due to menthol's ability to disturb the plasma membranes of bacteria, making them more permeable.