Zinc has been studied extensively for its role in treating and preventing the common cold, with several randomized controlled trials and meta-analyses conducted over the past few decades. Zinc is believed to exert antiviral effects by inhibiting the replication of rhinoviruses (the most common cause of colds) and by modulating the immune response. The most commonly studied preparations are zinc lozenges and zinc acetate or gluconate formulations.

A 2013 Cochrane review and a 2021 meta-analysis both found that zinc, when taken within 24 hours of symptom onset, can reduce the duration and severity of cold symptoms in otherwise healthy adults. The magnitude of the effect varies, with some studies showing a reduction in cold duration by 1 to 2 days. However, not all studies have found benefit, and the optimal dose, formulation, and frequency remain unclear. Gastrointestinal side effects (nausea, bad taste) are common, and intranasal zinc has been associated with rare cases of permanent anosmia (loss of smell).

Taken together, the evidence supports a modest benefit for zinc in shortening colds if started early, but results are inconsistent and benefit may not apply to all zinc preparations. Therefore, the use of zinc for colds is justified by scientific evidence, but with moderate strength of support (3/5).