Cineole (also known as eucalyptol) is a monoterpene and the main active constituent of eucalyptus oil. Its use for supporting or treating lung congestion is supported by a significant body of scientific evidence. Multiple randomized controlled trials and clinical studies have demonstrated that cineole exhibits mucolytic (mucus-thinning), anti-inflammatory, and bronchodilatory effects, which can be beneficial in respiratory conditions characterized by congestion, such as chronic obstructive pulmonary disease (COPD), asthma, and acute bronchitis.

For example, a double-blind, placebo-controlled trial published in Respiratory Medicine (2000) found that oral cineole significantly improved lung function and reduced exacerbations in patients with stable COPD. Other studies have shown that cineole can reduce sputum production and ease cough, likely due to its ability to decrease mucus viscosity and suppress airway inflammation by inhibiting cytokine production.

While cineole is also a component of traditional herbal remedies for congestion, its modern use is strongly underpinned by clinical research. Systematic reviews and meta-analyses generally support its safety and efficacy as an adjunctive therapy for respiratory tract diseases with congestion. However, it is not recommended as a sole therapy for severe conditions and should be used under medical supervision.