Caffeine (1,3,7-trimethylxanthine) is a plant alkaloid with usefulness as a therapeutic agent for hair loss prevention.  Caffeine is fat and water-soluble, easily added to preparations and absorbed through nearly all membranes.

When applied to the scalp, caffeine prolongs hair growth duration.  The growth-stimulating effect is due to its influence on growth factors that act on the hair follicle and its natural blood vessel dilating action.

One study that looked at effective concentrations on the hair follicle showed that very low concentrations of caffeine could increase the length of the hair shaft in less than 24 hours.  The success of caffeine preparations in reducing hair loss and encouraging hair growth is thought to be based on its ability to work simultaneously on two key growth-regulatory factors.