Essential oils are derived from plants. Their purpose is not exactly known, but some experts think that it could be a protective mechanism for the plant; others propose that they might attract pollinators. Peppermint is a popular essential oil used alone and in combination with other essential oils. Menthol is the main component of peppermint oil and is responsible for the noticeable cooling sensation. Menthol is extracted from plants or synthesized and can be found in throat lozenges, intranasal inhalers, lotions, pain creams, foods, and many other products.

Some people think that the cooling sensation from what is menthol is caused by mechanical damage by menthol crystals or by evaporation (like when you touch rubbing alcohol), but this is not the case. The cooling is an interaction on the molecular level with the calcium and channels in our bodies. Receptors that are responsible for feeling cold are triggered by menthol's effects on calcium and sodium channels and a cooling sensation occurs. There are calcium and sodium channels in cell membranes throughout our bodies, and that is why we can see systemic toxicity after swallowing or inhaling concentrated amounts of what is menthol. Serious effects include seizures, coma, and death.

Menthol can cause eye and skin irritation. When used on the skin, menthol is typically diluted into a "carrier oil", lotion, or other vehicle. If a high-percentage menthol product is applied to the skin, irritation and even chemical burns have been reported. There are a few reports of people being very sensitive to menthol and having severe skin reactions to even small amounts.

Products that contain menthol are flammable. In 2000, the FDA added a warning against putting products with what is menthol near a flame, in hot water, or in a microwave oven. There is additional warning about applying a menthol product to the skin and placing a heating pad over it.The heat increases absorption through the skin. In one case, putting a heating pad on a menthol rub resulted in tissue death. Some nasal inhalers contain menthol. The menthol makes us feel as though we are breathing easier, but it does not actually help with congestion. In fact, it appears that more inflammation develops.
Lastly, some researchers think that cigarettes with menthol might increase the risk of becoming addicted to smoking and might be more toxic than regular cigarettes. Nicotine replacement products used to stop smoking work for menthol cigarette smokers as well.