The mechanism by which zinc could impact the common cold is fairly straightforward: rhinovirus (the cause of the common cold) uses the ICAM-1 nasal epithelial cell receptor to latch on, and zinc ions block this. Since our bodies already usually have plenty of zinc on-board, the question is whether supplemental zinc can have an impact. In a JAMA clinical evidence synopsis, Das and Singh review 14 clinical trials of zinc in adults (n = 1781) and three in children.
Overall, common cold duration in adults was 7.5 days on placebo and 6.75 days when zinc supplementation was started within 24-48 hours of symptom onset, so folks got better about 1 day sooner. Disappointingly, symptom severity during illness was not alleviated.
Although not studied in adults, use of zinc as common cold prophylaxis in children has shown more impressive efficacy. When zinc supplements were provided at the onset of cold season, there was a 38% reduction in symptomatic colds compared to placebo (61.8% vs 38.2%). Whether similar benefits could be observed in adults has not been demonstrated. Similarly, at-risk groups among whom URI is particularly toxic (asthma, COPD, cystic fibrosis) have not been studied.
Many persons find zinc lozenges unpalatable, so whether the 1-day reduction in symptoms is worth the effort may be a personal decision. Guidelines from the American Academy of Family Physicians support the use of zinc within 24 hours of cold onset.
Source: Das RR, Singh M. Oral zinc for the common cold. JAMA 2014;311:1440-1441.