Eucalyptus oil is a standard ingredient in cough drops and cough syrups, as well as oils added to humidifiers. A standardized combination of eucalyptus oil plus two other essential oils has been studied for effectiveness in a variety of respiratory conditions. This combination therapy contains cineole from eucalyptus, d-limonene from citrus fruit, and alpha-pinene from pine. Because these oils are all in a chemical family called monoterpenes, the treatment is called “essential oil monoterpenes.”

DIM also has complex interactions with the hormone estrogen, which could lead to either positive or negative effects on cancer risk.

Most, though not all, studies indicate that oral use of essential oil monoterpenes can help acute bronchitis, chronic bronchitis, and sinus infections.2-7

For example, a double-blind, placebo-controlled trial of 676 people with acute bronchitisfound that 2 weeks treatment with essential oil monoterpenes was more effective than placebo and equally effective as antibiotic treatment for reducing symptoms and aiding recovery.5 Also, a 3-month, double-blind, placebo-controlled trial of 246 people with chronic bronchitis found that regular use of essential oil monoterpenes helped prevent the typical worsening of chronic bronchitisthat occurs during the winter.4

Additionally, in a double-blind, placebo-controlled study of about 300 people, use of essential oil monoterpenes improved symptoms of acute sinusitis.2

One study weakly indicates that essential oil monoterpenes may be helpful for colds in children.1

Essential oil monoterpenes are thought to work by thinning mucus.