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Depending on the total dosage of zinc and the composition of lozenges, zinc lozenges may shorten the duration of common cold episodes by up to 40 percent, according to a study published in the Open Respiratory Medicine Journal.
For treating the common cold, zinc lozenges are dissolved slowly in the mouth. Interest in zinc lozenges started in the early 1980s from the serendipitous observation that a cold of a young girl with leukemia rapidly disappeared when she dissolved a therapeutic zinc tablet in her mouth instead of swallowing it. Since then over a dozen studies have been carried out to find out whether zinc lozenges are effective, but the results of those studies have diverged.
Dr. Harri Hemila of the University of Helsinki in Finland, carried out a meta-analysis of all the placebo-controlled trials that have examined the effect of zinc lozenges on natural common cold infections. Of the 13 trial comparisons identified, five used a total daily zinc dose of less than 75 mg and uniformly those five comparisons found no effect of zinc. Three trials used zinc acetate in daily doses of over 75 mg, with the average indicating a 42 percent reduction in the duration of colds. Five trials used zinc salts other than acetate in daily doses of over 75 mg, with the average indicating a 20 percent decrease in the duration of colds.
In several studies, zinc lozenges caused adverse effects, such as bad taste, but there is no evidence that zinc lozenges might cause long term harm. Furthermore, in the most recent trial on zinc acetate lozenges, there were no significant differences between the zinc and placebo groups in the occurrence of adverse effects although the daily dose of zinc was 92 mg. Hemila concluded that, "Since a large proportion of trial participants have remained without adverse effects, zinc lozenges might be useful for them as a treatment option for the common cold."