The discovery that zinc lozenges might shorten the duration of the common cold occurred by chance, when common cold symptoms disappeared in a 3-year old girl a few hours after she slowly dissolved a zinc tablet in her mouth rather than immediately swallowing it
Her father, George Eby, conducted the first randomised controlled trial of zinc lozenges.
He showed that zinc gluconate lozenges, providing 207mg per day of elemental zinc, significantly shortened the duration of the common cold. That was in 1984.
Since that time, numerous trials have evaluated zinc lozenges for the common cold but have produced mixed results.
Trial designs have varied greatly and used different zinc salts at different dosages. Eby proposed that the variation in free zinc ions in the oropharyngeal region may explain the disparate results of clinical trials and hypothesised that zinc acetate might be more effective than zinc gluconate because acetate binds less strongly to zinc ions.
In 2017 in the Journal of the Royal Society of Medicine, Harri Hemilä compared the efficacy of zinc acetate lozenges with zinc gluconate lozenges and examined dosage effects in the treatment of the common cold.
The meta-analysis included seven placebo-controlled trials including 575 patients with the common cold. The intervention in all trials was zinc lozenges delivering 80–207mg per day of elemental zinc. The primary outcome measure was duration of the common cold.
The pooled estimate in all seven trials showed a reduction in the common cold duration by 33% (95% CI, 21%-45%).
The individual trials showed zinc lozenges to shorten colds by 1.3–4.0 days, with zinc acetate shortening cold duration by an average of 2.7 days.
The efficacy of the two salts did not differ significantly. The efficacy of different dosages also did not differ significantly, with the therapeutic effect reached with dosages of 80–92mg of elemental zinc per day. There was no evidence that dosages higher than 100mg per day offered any additional advantage.
The author of this analysis highlights the importance of providing zinc lozenges at an adequate dosage level and the importance of avoiding formulations that contain substances that bind zinc, including citric acid, tartaric acid, sodium bicarbonate, hydrogenated oil, cottonseed oil, soy lecithin, mannitol and sorbitol.
This was a well-conducted meta-analysis that included data collected from than three decades by six different research groups.
The author concludes that the evidence is strong to recommend zinc lozenges (either as zinc acetate or zinc gluconate) at dosages of 80mg per day to reduce the duration of the common cold.