Because of its cooling effect, eucalyptus essential oil may be used to lower the body's temperature when fever occurs. The body produces heat to fight off the infectious agents trying to take over your system. Eucalyptus oil can also act as a vasodilator. A vasodilator is taken to relax the blood vessels, which allows the blood to flow more smoothly. You won't need to use a lot of the oil since the fever itself normally causes blood circulation to increase by the rise in body temperature. Several websites suggest mixing it with other essential oils to treat the fever more effectively.