Eucalyptus is an evergreen tree native to Australia but is cultivated worldwide. The plant’s leaves—and the oil that is steam-distilled from them—are used medicinally.¹

Eucalyptus was first used by Australian aborigines, who not only chewed the roots for water in the dry outback but used the leaves as a remedy for fevers. In the 1800s, crew members of an Australian freighter developed high fevers, but were able to successfully cure their condition using eucalyptus tea. Thus, eucalyptus became well known throughout Europe and the Mediterranean as the Australian fever tree. Early 19th century Eclectic physicians in the United States not only used eucalyptus oil to sterilize instruments and wounds, but recommended a steam inhalation of the vapor of its oil to help treat asthma , bronchitis , whooping cough, and emphysema.²

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.¹⁷ 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 .¹⁸ In test tube studies, eucalyptus species have been shown to possess antibacterial actions against such organisms as Bacillus subtilis,¹⁹ as well as several strains of Streptococcus.²⁰ 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.²¹ 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.²² 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.²³ Also, in a double-blind trial, a eucalyptus-based rub was found helpful for warming muscles in athletes.²⁴ This further suggests eucalyptus may help relieve minor muscle soreness when applied topically, though studies are needed to confirm this possibility.

Eucalyptus oil (0.05–0.2 ml per day) can be taken internally by adults.²⁵ It should always be diluted in warm water before consuming. For local applications, 30 ml of the oil can be mixed in 500 ml of lukewarm water and applied topically as an insect repellent or used over the temporal areas of the forehead for tension headaches. As an inhalant, add a few drops of eucalyptus oil to hot water or a vaporizer. Deeply inhale the steam vapor. For eucalyptus leaf preparations, an infusion of 2–3 grams of the chopped leaves may be boiled in 150 ml of water and taken two times per day. Eucalyptus oil needs to be used very cautiously since as little as 3.5 ml of the oil taken internally has proven fatal.²⁶ It is best for people to discuss internal use with a qualified healthcare professional.

Warning: Eucalyptus oil needs to be used very cautiously since as little as 3.5 ml of the oil taken internally has proven fatal. It is best for individuals to discuss internal use with a qualified healthcare professional.

Although there are no known reports of drug interactions, the German Commission E monograph suggests that because eucalyptus oil may activate certain enzyme systems in the liver, it may potentially weaken or shorten the action of some medications, including pentobarbital, aminopyrine, and amphetamine.^{27 , 28}

Side effects from the internal use of eucalyptus can include nausea, vomiting, and diarrhea . Eucalyptus oil should not be used by infants and children under the age of two, especially near the face and nose, due to the risk of airway spasm and possible cessation of breathing.²⁹ The oil may aggravate bronchial spasms in people with asthma and should not be taken internally by those with severe liver diseases and inflammatory disorders of the gastrointestinal tract and kidney.^{30 , 31} Whole-body application of eucalyptus oil (double-distilled, containing 80–85% cineole oil) resulted in severe nervous system toxicity in a six year old girl.^[REF] In a case report, a 4-year-old girl suffered a seizure after application of a eucalyptus oil preparation to the hair and scalp for the treatment of head lice.³² Eucalyptus should not be used in large amounts by people with low blood pressure as it may cause a further drop in blood pressure.³³ The safety of eucalyptus oil has not been established in pregnant or nursing women.