Consistency of the summarized study with previous studies

The results of the summarized study are consistent overall with other meta-analyses on zinc and the common cold. For example, in other meta-analyses, researchers reported reductions in the duration of the common cold by approximately 1–3 days from zinc supplementation.^[1]^[2]^[3]^[1]^[4]^[5]^[6] For context, the average duration of a cold is 7–10 days, though symptoms can persist for 3 weeks.^[7]^[6]

In addition, other meta-analyses have also shown an increased risk of nonserious adverse events from zinc supplementation for participants with colds.^[5]^[3]^[2]

Despite the popularity of the lozenge formulation of zinc, the summarized study suggests that they are equally as effective as nasal sprays for reducing cold duration, although the dosage of nasal sprays was much lower. Furthermore, a 2015 meta-analysis found that lozenges did not have a stronger effect on throat symptoms compared to nasal symptoms, despite lozenges directly exposing the throat to zinc ions.^[2] Also similar to the summarized study, the zinc dosage also did not have different effects on cold duration in a 2016 meta-analysis.^[6]

Also like the summarized study, other studies found no effect of zinc on reducing cold severity nor on preventing colds.^[1]^[3]^[5] One narrative review concluded that zinc supplementation reduced cold severity, but it did not evaluate study bias or the certainty level of the evidence.^[8]

Study strengths

The summarized study is bolstered by the inclusion of both adults and children in the participants,^[1]^[5] examining a wider variety of formulations of zinc,^[2]^[3]^[1]^[4] and conducting a meta-analysis and assessing the level of certainty of the evidence.^[8] In addition, one of the other reviews was withdrawn due to errors and plagiarism.^[4] Furthermore, the summarized study serves as a good update to the literature because the other meta-analyses were published between 2004 and 2021.

Mechanisms

Zinc may improve the common cold because it is required for the maintenance and development of various immune cells.^[9] Also, zinc may interfere with the binding and replication of the viruses responsible for colds.^[8]

Dosages

For adults, the Recommended Dietary Allowance (RDA) of zinc is 8–11 mg per day, and the Tolerable Upper Intake Level (TUIL) is 40 mg per day.^[10] Dosages were most commonly greater than 45 mg per day in the summarized study, so it is unsurprising there was an increased risk of adverse effects reported from zinc supplementation, especially at dosages greater than 85 mg per day. Although several RCTs did use dosages of 5–15 mg per day, it is difficult to determine whether these dosages would be effective, given that the subgroup analysis showing that high-dosage vs. low-dosage didn’t affect cold duration was based on a much higher threshold of 85 mg per day. However, given that nose sprays were equally as effective as lozenges for reducing cold duration, and the nose sprays had a much lower dosage (the majority of the studies used 2.1 mg/day), it is certainly feasible that low-dosage zinc may be effective for reducing common cold duration when used this way.

Ultimately, the summarized study and much of the other literature in the field currently shows that zinc may be effective for reducing the duration of the common cold, but it may cause symptoms such as taste irregularities and stomach upset.

Does zinc help with the common cold? Original paper