The major constituent in eucalyptus leaves is a volatile oil known as eucalyptol (1,8-cineol). In order to provide an effective expectorant and antiseptic action, the leaf oil should contain approximately 70–85% eucalyptol.24 Eucalyptus oil is said to function in a fashion similar to that of menthol by acting on receptors in the nasal mucosa, leading to a reduction in symptoms such as nasal congestion.25 In test tube studies, eucalyptus species have been shown to possess antibacterial actions against such organisms as Bacillus subtilis,26 as well as several strains of Streptococcus.27 These actions have not been researched in human clinical trials.
Peppermint (10 grams) and eucalyptus oil (5 grams) in combination, applied topically to the forehead and temples for three minutes with a small sponge, have been shown to be helpful as a muscle relaxant (but not for pain relief) in people with tension headaches.28 A eucalyptus oil extract containing 50% p-methane-3,8-diol (PMD) as the active ingredient has been shown to be effective in protecting human volunteers from various types of biting insects.29 On human forearms, it was determined that the eucalyptus extract was nearly as effective as a 20% solution of diethyltoluamine (used in many insect repellents) in repelling bites of the Anopheles mosquito (the insect that spreads malaria) for up to five hours. The eucalyptus extract was also effective at repelling flies (94%) and midges (100%) for up to six hours.
A preliminary study suggests the combination of eucalyptus and menthol as a nasal inhalant is helpful in cases of mild to moderate snoring.30 Also, in a double-blind trial, a eucalyptus-based rub was found helpful for warming muscles in athletes.31 This further suggests eucalyptus may help relieve minor muscle soreness when applied topically, though studies are needed to confirm this possibility.