Elderberry

Sambucus nigra, the European black elder, owes its activity to an unusually dense concentration of anthocyanins, principally cyanidin-3-glucoside and cyanidin-3-sambubioside. In vitro these compounds bind viral haemagglutinin and interfere with the ability of influenza virions to attach to and enter host cells, while also modulating cytokine release from monocytes.

The clinical picture is better than most botanical antivirals but has real limits. A 2019 meta-analysis of randomised trials found elderberry substantially reduced upper respiratory symptom duration, with the largest effects when supplementation began within 24-48 hours of onset. The two most cited flu trials showed symptom resolution roughly four days earlier than placebo, though both were small. A larger 2020 trial in laboratory-confirmed influenza found no benefit, which tempers enthusiasm and suggests the effect may be smaller than early studies implied.

Safety hinges entirely on preparation. Raw elderberries, leaves, bark and stems contain cyanogenic glycosides — sambunigrin among them — that cause nausea, vomiting and diarrhoea, with documented poisoning outbreaks from unheated juice. Commercial extracts are heated or otherwise processed to remove these compounds and have an excellent safety record. Because elderberry can increase certain inflammatory cytokines in vitro, people on immunosuppressants or with autoimmune disease should check with their clinician.