Slippery elm (Ulmus rubra) has a long history of use in traditional herbal medicine, particularly among Native American tribes and later in North American folk medicine. Its inner bark contains mucilage, a gel-like substance that, when mixed with water, forms a soothing, slippery consistency. This mucilage is thought to coat and soothe mucous membranes of the throat and respiratory tract, which is why slippery elm has traditionally been used as a remedy for coughs, sore throats, and bronchitis.

The evidence supporting slippery elm's use for bronchitis is primarily anecdotal and derived from traditional practices rather than modern clinical trials. Historical texts and herbal guides often list slippery elm as a demulcent, meaning it can relieve minor pain and inflammation of mucous membranes. The idea is that this coating action may help alleviate the irritation and coughing associated with bronchitis.

However, scientific studies specifically evaluating its efficacy for bronchitis are lacking. While the mucilage does have well-documented physical properties that can soothe irritated tissues, there is insufficient clinical research to substantiate its effectiveness for bronchitis beyond its use as a general throat soother. Thus, while its use is rooted in tradition and there is some plausible mechanism of action, robust scientific validation is currently lacking.