Echinacea purpurea has been traditionally used by Native Americans and later adopted in Western herbal medicine to help prevent and treat the common cold. Over the past few decades, its use as an "immune booster" has become widespread, prompting a significant amount of scientific research. Laboratory studies suggest that echinacea extracts can stimulate various immune cells and may have mild antiviral properties, particularly against some respiratory viruses.

Clinical trials investigating echinacea’s effectiveness for the prevention and treatment of colds have produced mixed results. Some randomized controlled trials (RCTs) and meta-analyses suggest a modest reduction in the duration and severity of cold symptoms when echinacea is used at the onset of illness. However, other studies find no significant benefits compared to placebo. Methodological differences, variations in echinacea species/preparations, and small sample sizes complicate interpretation.

Overall, the evidence does not definitively support echinacea as a reliable antiviral or cold remedy, but neither does it wholly refute its potential. Most authoritative reviews (such as those by Cochrane and the NIH) conclude that echinacea might have a small beneficial effect, but results are inconsistent, and further high-quality studies are needed. The safety profile is generally good, with mild side effects (such as gastrointestinal upset or allergic reactions) occurring in a minority of users.