Zinc supplements provided modest benefits in preventing and treating common symptoms of respiratory tract infections (RTIs) in adults, according to a meta-analysis published in BMJ Open.

Cheat sheets: Evidence-based medicine 101

Meta-analysis details

For the analysis, researchers at Western Sydney University looked at 28 randomized controlled trials (RCTs) involving 5,446 adults. The trials were published in 17 English and Chinese research databases as recently as August 2020. Notably, the researchers disclosed that none of the trials used in the meta-analysis studied zinc supplements for Covid-19 prevention and treatment.

The studies included in the meta-analysis looked at multiple forms of zinc supplements, including lozenges, nasal sprays, and gels. Of the 28 studies, only three did not include a placebo control.

The researchers acknowledged several limitations to the research, including the possibility of publication bias in the analysis. However, the group wrote, "While publication bias was not strongly suspected, visual inspection of funnel plots are necessarily subjective and a statistical test for hazard ratios was not performed." They also highlighted the need for additional research to better understand how zinc fights viral infections. 

Zinc supplements are a 'viable' alternative for treating RTIs 

Overall, the meta-analysis found "some evidence suggesting zinc might prevent RTI symptoms and shorten duration" among adult populations who are not zinc deficient. Specifically, the researchers concluded that based on the research, zinc treatments appeared to "shorten the duration of symptoms and reduce day three symptomatic severity, but not overall daily symptom severity."

On average, according to the researchers, RTI symptoms improved two days earlier for individuals who took zinc treatments compared to the individuals who took placebos—and 19% of adults were more likely to still experience symptoms a week after their illness began if they did not take zinc supplements.

The researchers also found that individuals who used zinc treatments saw "minor and infrequent" side effects, MedPage Today reports. However, higher doses used to treat active infections resulted in gastrointestinal discomfort for some individuals, and sublingual lozenges caused irritation in the mouth for some.

Even though some of the zinc supplements in the studies had "unclear" doses and posed a risk of minimal side effects, researchers concluded that "[t]he marginal benefits, strain specificity, drug resistance and potential risks of other over-the-counter and prescription medications makes zinc a viable 'natural' alternative for the self-management of non-specific [RTIs]."

The researchers added that zinc supplements may provide medical professionals and patients "a management option for patients who are desperate for faster recovery times and might be seeking an unnecessary antibiotic prescription." (Hampson, The Independent, 11/2; Gever, MedPage Today, 11/1)