As cold and influenza season begins, many consumers rely on nonprescription products marketed to support immune health and help prevent or manage respiratory illnesses. Among the numerous dietary supplements available in community pharmacies, echinacea and elderberry remain 2 of the most recognizable herbal ingredients promoted for immune support. However, these products represent only a portion of the herbal preparations traditionally used for respiratory health.

Beyond echinacea and elderberry, several herbal products with long histories of use in Asian and European traditional medicine may be less familiar to US consumers. As interest in complementary medicine continues to grow, pharmacists should be familiar with the available evidence on the efficacy, safety, adverse effects, and potential drug interactions associated with these products.

Sea Buckthorn

Sea buckthorn (SBT), or Hippophae rhamnoides (H rhamnoides) is a flowering shrub in the Elaeagnaceae family native to cold-temperate Eurasia. The plant produces clusters of bright orange berries and contains numerous bioactive constituents, including flavonoids, carotenoids, polyunsaturated fatty acids, vitamins, and minerals. Most notably, the fruit has high contents of vitamin C, vitamin E, vitamin B12, and zinc. Experimental studies of SBT preparations have demonstrated antioxidant, cytoprotective, antimicrobial, and anti-inflammatory activity, although these findings do not necessarily translate into clinical benefit.

In the United States, people commonly use dietary supplements and complementary therapies such as vitamin C, zinc, honey, echinacea, and elderberry during cold and influenza season. Researchers have studied elderberry for its potential effects on the duration and severity of upper respiratory symptoms, but the available evidence remains limited and uncertain. Echinacea has similarly been evaluated for the prevention and treatment of upper respiratory infections, with studies reporting inconsistent findings regarding its clinical effectiveness. Like these familiar products, SBT contains bioactive compounds with antioxidant and anti-inflammatory properties that have prompted interest in its potential role in respiratory health.

Clinical Evidence in Respiratory Illness

Clinical evidence suggests that SBT-containing preparations may help reduce the risk of respiratory infections. In a randomized, double-blind, placebo-controlled study, 50 participants received either a nutritional supplement containing sea buckthorn, honey, and propolis or placebo for 1 month during peak respiratory virus season. Researchers monitored participants for respiratory symptoms and evaluated suspected infections using rapid antigen tests for COVID-19 and influenza.

Among participants who completed the study per protocol, the combination supplement was associated with an approximately 71.4% relative reduction in the risk of respiratory infection compared with placebo.⁵ Although these findings are promising, the small sample size, per-protocol analysis, and use of a combination product limit the ability to determine the independent effect of SBT. Larger controlled studies evaluating standardized SBT preparations alone are needed to determine benefits in a larger patient population.

Preclinical Coronavirus Research

Preclinical studies have also evaluated the potential antiviral activity of SBT and some of its bioactive compounds. In an in vitro study by Zhan et al, the flavonoids quercetin and isorhamnetin were tested for their ability to inhibit entry of SARS-CoV-2 spike pseudovirus into ACE2-expressing cells. At the tested concentration, isorhamnetin reduced pseudovirus entry to about 47.7% of the control level and demonstrated no significant cytotoxicity, whereas quercetin did not significantly reduce viral entry.

Additional in vitro research by Al Ibrahim et al evaluated several plant extracts for activity against coronaviruses. Extracts derived from H rhamnoides demonstrated antiviral activity against HCoV-229E and SARS-CoV-2 in cell-based models; subsequent fractionation identified cinnamoyl triterpenoids from SBT with antiviral activity.

These conclusions suggest that SBT contains compounds with potential antiviral properties; however, it is important to note that these studies were conducted in laboratory models, and the concentrations, formulations, and biological responses observed in vitro may not produce similar effects in humans. Clinical studies are needed to determine whether these outcomes translate into meaningful prevention or treatment of coronavirus infections.

Preclinical Influenza Research

SBT leaf extracts have also demonstrated anti-influenza activity in vitro. In a study by Enkhtaivan et al, several SBT extracts inhibited influenza A and B viruses in cell-based assays. Some extracts showed lower half-maximal inhibitory concentration (IC₅₀) values than oseltamivir, used as a laboratory comparator.

Although these findings are noteworthy, they do not show that SBT is more effective than oseltamivir in clinical practice. The study evaluated plant extracts in cell-based assays rather than standardized SBT preparations administered to patients. The study did not assess pharmacokinetics, bioavailability, metabolism, dosing, or patient outcomes. Controlled human studies are needed to determine whether these laboratory findings are clinically relevant.

Safety and Potential Drug Interactions

Although SBT has shown promising biological activity, clinicians should consider its potential effects on platelet function when counseling patients. An in vitro study by Olas et al evaluated the effects of a phenolic-rich fraction of SBT fruit on platelet adhesion. Platelets exposed to SBT phenolic fractions showed decreased adhesion compared with untreated platelets after thrombin activation.

A small crossover study by Johansson et al evaluated sea buckthorn berry oil supplementation in 12 healthy men. Participants received either sea buckthorn berry oil or fractionated coconut oil for 4 weeks. Sea buckthorn supplementation was associated with reductions in the rate and maximum extent of adenosine diphosphate–induced platelet aggregation.

These findings suggest that SBT may affect platelet function. However, SBT has not been linked to increased clinical bleeding, and available human evidence is limited. Nevertheless, caution may be warranted in patients taking anticoagulant or antiplatelet medications. Pharmacists should ask patients about dietary supplement use and consider the potential for additive effects on platelet function when counseling patients interested in SBT products.

Implications for Pharmacists

Sea buckthorn contains numerous biologically active compounds and has demonstrated antioxidant, cytoprotective, antimicrobial, anti-inflammatory, and antiviral activity in experimental studies. However, much of the evidence supporting its potential use for respiratory illness remains preclinical. Available human evidence is limited, and the principal clinical trial evaluated SBT as part of a combination product rather than as an independent intervention.

Pharmacists are well positioned to help patients distinguish promising laboratory findings from established clinical efficacy with regard to all herbal and dietary supplementation. Advise patients interested in SBT that current evidence is insufficient to recommend it as a substitute for vaccination, antiviral therapy, or other evidence-based strategies to prevent or treat influenza, COVID-19, or other respiratory infections.

Pharmacists should also review concomitant medications, particularly anticoagulants, antiplatelets, and antihypertensives, when counseling patients about SBT supplementation.

Additional well-designed clinical trials using standardized SBT preparations are needed to establish appropriate dosing, clinical efficacy, and long-term safety. Until these data are available, SBT should be considered a supplement with promising preclinical activity rather than an established therapy for respiratory infection.