Zinc shortens a cold

Zinc taken once a cold has already started may shorten it, and by how much is not settled. The largest and most recent synthesis, a 2024 Cochrane review covering 34 trials and 8,526 participants, found that when zinc was used as a treatment, colds ended on average 2.37 days sooner than on placebo, with a confidence interval running from 4.21 days to 0.53 days. That average pools 8 trials and 972 participants: five trials of lozenges, one of them in children, and three trials of zinc sprayed or applied inside the nose. Cochrane rated the finding low certainty. Heterogeneity was 97 percent, so the trials being averaged disagreed with one another almost completely, and when the review kept only the three trials at low risk of bias in how participants were assigned to zinc or placebo, the difference fell to 0.57 days, with a confidence interval running from 1.30 days shorter to 0.16 days longer.

The lozenge trials differ in dose and in what else the lozenge contained. Analyses restricted to zinc acetate lozenges delivering 80 to 92 mg of elemental zinc a day report reductions of about 2.7 to 2.9 days against a 7 day average cold. A 2020 randomized, double-blind trial of a commercially available zinc acetate lozenge, 13 mg per lozenge taken six times a day for five days, found no difference in recovery rate over 10 days of follow-up. The doses in the trials reporting large effects run roughly two to seven times the tolerable upper intake level of 40 mg a day for adults, taken as repeated lozenges through the waking day, and non-serious side effects were more common on zinc than on placebo across 16 trials and 2,084 participants. The NIH Office of Dietary Supplements fact sheet on zinc documents copper deficiency and interactions with other drugs at sustained high zinc intake.

The other pooled estimates land in the same region. A 2012 meta-analysis of 17 trials and 2,121 participants found colds 1.65 days shorter overall and 2.63 days shorter in adults, with no significant effect in children, and heterogeneity of 95 percent. A 2020 systematic review reported 2.25 days shorter when zinc was given on its own. Each of these is an average across trials that do not agree with each other. A 2024 re-analysis of the Cochrane data set the nasal trials aside, added a 1996 lozenge trial that the review had left out of its average, and measured the effect as a share of the cold's length instead of in days. Across five lozenge trials in adults, colds were 37 percent shorter on zinc, with a confidence interval of 27 to 46 percent, and the one lozenge trial in children, which found no difference between zinc and placebo, was kept apart from the adult trials. The re-analysis reuses the trials the review had already selected, and its first author also wrote the high-dose acetate analysis and ran the 2020 trial described above.

Duration is not the same thing as severity, and severity did not move. The 2024 review found no difference in global symptom severity across 2 trials and 261 participants, rated very low certainty, and no difference in the risk of a change in global symptom severity in a single 114 participant trial. A cold that ends sooner and a cold that feels milder are two different results, and only the first has support here.

Two things make these trials hard to run cleanly. Zinc lozenges taste strongly of zinc, so participants can often tell which arm they are in, and a 2012 review named placebo matching as a specific problem in this literature. Several of the older lozenge trials were paid for by the companies selling the lozenges, recorded sponsor by sponsor in that review's own table of included trials. The 2024 review judged most of the trials it included to be at unclear or high risk of bias in at least one domain.

Route matters. Zinc gels and swabs used inside the nose have been linked to long-lasting and sometimes permanent loss of the sense of smell, and the FDA warned consumers away from several intranasal zinc cold products in 2009. Three of the eight trials behind the 2024 average tested zinc inside the nose, so that average is not a figure for lozenges alone, and the review saw no apparent difference in cold length between the nasal trials and the lozenge trials. Everything else in this summary concerns zinc taken by mouth.

Whether taking zinc regularly prevents a cold in the first place is a separate question with its own set of trials, and it is not answered here.