Effectiveness
A statistical analysis which pooled seven randomized trials with 575 participants with naturally acquired colds concluded that there was no evidence that zinc acetate and zinc gluconate lozenges differed in their efficacy, there being on average 33% shorter colds in the zinc lozenge groups. [3] In addition, the analysis found that there was no evidence that zinc doses over 100 mg/day led to greater efficacy in the treatment of the common cold.
Based on individual patient data of 199 common cold patients, zinc acetate lozenges indicated that such lozenges may shorten common cold duration by 36% (2.7 days), [4] and increased the recovery rate by rate ratio 3.1. [5]
Two statistical analyses highlighted that the effect of zinc lozenges varied depending on the initial length of the cold, with longer colds experiencing substantially greater reduction in duration that short colds. [6] [7]
A 2021 systematic review and meta-analysis on respiratory tract infections found that zinc modestly reduced symptom severity by day 3 and shortened illness duration by about two days, though the evidence was of low to very low certainty. Zinc offered minimal benefit in preventing infections and was associated with an increased risk of mild side effects, such as nausea and irritation. [8]
A 2024 Cochrane Review found little to no evidence that zinc prevents the common cold or reduces symptom severity, though it may modestly shorten the duration of symptoms. Zinc lozenges were associated with a reduction in cold duration, but the evidence was of low certainty and varied across studies. [9]
Unfortunately, the 2024 Cochrane review [9] has a number of severe flaws. First, the mean difference (i.e. effect on days) is not an appropriate scale in the analysis of effects on common cold duration; the effect on the percentage scale fits the data much better. [6] [10] Second, the Cochrane review stated that they will pool trials only if they “were similar enough for pooling to make sense”. However, Analysis 9.1 in the Cochrane review includes both zinc lozenge trials and nasal zinc administration trials, although they are not similar treatments. Furthermore, one of the included zinc lozenge trials administered 190 mg/day zinc whereas one nasal zinc trial administered just 0.046 mg/day. Pooling two trials with a 4,300-fold difference in the dose does not make sense. Third, the Cochrane authors dis not justify their exclusion of the Mossad et al. (1996) trial from their Analysis 9.1, even though it was a placebo-controlled randomized trial. These problems have been described in detail. [11] [12] [13]
Interactions
Some lozenge formulations do not contain enough zinc to effectively reduce the lengths of colds; and some of them contain ingredients that bind zinc, like citric acid, which prevent the zinc from working. [11] [14]
Availability of proper zinc lozenges
Although there is evidence that properly composed zinc lozenges can shorten the duration of colds, it is not easy for common cold patients to find effective lozenges. Eby wrote in his 2010 review that “Zinc lozenges marketed in the United States appear to compete based upon taste rather than efficacy… Of the 40 different brands of over-the-counter zinc lozenges and many variations of them currently available in the US, very few—based upon this analysis and ingredients listed on their labels—appear to release useful amounts of iZn [free zinc ions] regardless of total zinc content, and none of them can be considered as a cure for common colds. With several exceptions, nearly all appear likely to have a null effect on colds.” [14] . However, this may be overcome with advice from health practitioners to seek lozenges that have appropriate levels of zinc and do not contain citric acid.
Safety
High doses of zinc have been given to patients with various diseases for several months without concerns, and zinc is a standard treatment for Wilson's disease, which usually means taking high doses long-term. [12] In the treatment of Wilson's disease, 150 mg/day of zinc has had an excellent safety profile, though it has caused gastric irritation in 5–10% of patients. Thus, it seems highly unlikely that about 100 mg/day of zinc in the zinc lozenge trials, for 1–2 weeks would cause long-term severe adverse effects.
In the reported trials zinc lozenges have caused bad taste for some patients, but a substantial proportion of patients while a substantial proportion of patients have not experienced adverse effects, a few have discontinued trials because of them. [11]
There have been several cases of people using zinc nasal sprays and suffering a loss of sense of smell. [1] In 2009 the US Food and Drug Administration issued a warning that people should not use nasal sprays containing zinc. [1]
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