Menthol is a naturally occurring chemical compound derived from the oil of various mint plants. Numerous studies have been done showing that menthol serves a number of purposes in the area of application. These include working as a counter-irritant to increase circulation through vasodialation, provide a cooling effect through the activation of TRPM8 receptors in the skin, and facilitating increased absorption of other compounds, such as methyl salicylate (found in many liniments) which has analgesic (pain-relieving) effects. (1) Other benefits of menthol include acting itself as a local analgesic through selective activation of k-opioid receptors,(2) as well as by blocking voltage-sensitive sodium channels within the tissue. (3)

Menthol concentration in veterinary liniments on the market in both liquid and gel form vary significantly from around 1% to 7%. There is a study that was done comparing menthol concentration to effectiveness, and it found that concentrations as low as .5% and as high as 10% actually produced less cooling sensation on the human test subjects than those with a more moderate concentration.  This means you're looking for a menthol concentration ideally around the 4-7% range for maximum effectiveness. (4)

A word of caution on that note; while any brand of liniment can cause this, the enhanced cooling "tingly" sensation caused by the target menthol concentrations, while more effective, may also trigger a more excitable reaction in your horse for a couple minutes when you first apply it. This can happen with any method of application, but most noticeably when applied as a body wash.

Pay attention to the blend of other ingredients found in the various liniments on the market. You'll see many of them with the characteristic green color, which is almost always achieved using artificial coloring. This may or may not be a big deal for you personally and if so look for a liniment that leaves the coloring out, there are several on the market. Other ingredients should include some form of skin moisturizers and healing agents. There should be various rubefacients, which are substances that cause the blood vessels near the skin to open up, or vasodilate, to allow increased circulation to the affected area. Finally, there are usually some emulsifiers, humectants, and other agents which help all the ingredients become and stay blended together.

As with all therapeutic treatment regimens, it is wise to consult with your veterinarian before you begin treatment. Package directions should also be followed, as some manufacturers may or may not recommend various methods of application. This includes all listed uses below.