A Review of the Properties of Clinically Evaluated Plant-Derived Agents in the Treatment of Respiratory Infections

Background: The use of plant-derived agents is a common approach in integrative care for respiratory conditions. However, the evidence of clinical trials has not yet been comprehensively presented. Aim: To summarize the antibacterial, antiviral, immunomodulatory, antioxidant, and expectorant properties of Echinacea spp., Pelargonium sidoides, Hedera helix, Thymus vulgaris, Althaea officinalis, Sambucus nigra, Zingiber officinale, and Curcuma longa, and to evaluate the evidence level from clinical trials (CTs) involving these agents in patients with respiratory tract infections (RTIs). Methods: We conducted a literature search using the PubMed database focusing on clinical studies of plant-derived agents in upper and lower RTIs. PRISMA-based reporting elements were used only as a guiding tool for comprehensibility of the literature search (Reporting Items for Systematic Reviews and Meta-Analyses guidelines). Results: A summary and structured overview of the properties of these most-cited plant-derived agents in the literature, in the context of RTIs, was provided. A total of 94 reports met the eligibility criteria and were included in our review. Of these, 66 reported randomized and placebo-controlled trials investigating the efficacy and tolerability of these adjuncts in patients with RTIs. The non-randomized and uncontrolled trials were 22. Sufficient evidence to be regarded as an appropriate treatment to reduce the severity and duration of RTIs was found for all discussed plant-derived agents. Robust evidence available was found for Echinacea spp., Pelargonium sidoides, Sambucus nigra, Curcuma longa and Zingiber officinale. Conclusions: Regarding other plant-derived agents reported in the traditional medicine for the treatment of RTIs, further research is needed to clarify the evidence gaps.