Zinc and the common cold
Zinc lozenges (frequently zinc acetate or zinc gluconate) have been proposed as a treatment for the common cold. The biological mechanism of the effect is not clear, but the benefit of zinc lozenges seems to be caused by local effects in the oropharynx region, since nasal administration of zinc has also shortened the duration of colds.
Research to date has not been conclusive, but has suggested that zinc tablets lessen the symptoms of a cold while causing possible side-effects such as nausea. The benefits of zinc as a cold treatment have been described as "very minor".
Zinc has been known for many years to have an effect on cold viruses in the laboratory.
Research into the effect of zinc lozenges on colds between 1984 and 2009 produced a wide range of differing results. It has been hypothesized that these differences were due in part to differences in the lozenges' constituents, mainly the amount of the active ingredient, ionic zinc (iZn). For example, the amount of ionic zinc ranged from 0 to 100%, depending upon the lozenge ingredients in the trials. More recently, the evidence from trials has been reviewed to try and assess the evidence of zinc's effectiveness as a cold cure but the results, while suggestive of a positive effect, have not been conclusive, since no consideration was given to the amount of the active ingredient (iZn) in the lozenges.
In some studies zinc lozenges caused acute adverse effects, such as bad taste, but none of the studies reported long term harm. Furthermore, many of the adverse effects, in particular regarding taste, may be caused by the specific lozenge composition and probably do not reflect the effects of zinc ions themselves. For example, zinc gluconate with dextrose or sugar becomes extremely vile in taste several weeks after manufacture, while zinc acetate dihydrate in a 1:100 mixture with dextrose by weight is permanently pleasant tasting. The most recent trial on zinc acetate found no significant differences between the zinc and placebo groups in the recorded adverse effects with a 92 mg daily dose of zinc.
According to WebMD, there have been several cases of people using zinc nasal sprays and suffering a loss of sense of smell; for this reason the US Food and Drug Administration has issued a warning that people should not use nasal sprays containing zinc.
All reviews to date have suffered from the variability of the studies which have been assessed; future large-scale randomized trials are necessary to determine whether the benefits from zinc treatment for colds outweigh the disadvantages. 
The most recent meta-analysis on zinc lozenges and the common cold found no difference in the effects of zinc acetate lozenges on diverse respiratory symptoms. Although zinc lozenges most probably lead to highest concentration of zinc in the pharyngeal region, a subsequent meta-analysis showed that the effects of high-dose zinc acetate lozenges did not significantly differ in their effects on pharyngeal and nasal symptoms. The duration of nasal discharge was shortened by 34%, nasal congestion by 37%, sneezing by 22%, scratchy throat by 33%, sore throat by 18%, hoarseness by 43%, and cough by 46%. Zinc lozenges shortened the duration of muscle ache by 54%, but there was no significant effect on the duration of headache and fever.
A 2012 systematic review suggested that "zinc formulations may shorten the duration of symptoms of the common cold", but that further research was needed and that possible adverse effects needed to be studied.
A 2009 review to consider the effects of the amount of the active ingredient, ionic zinc (iZn), showed that results varied in a direct relationship to the amount of iZn in the lozenges and total daily intake of iZn. Those studies that had zero or little iZn showed no results or even worsened results. Studies that used lozenges releasing large amounts of iZn showed strongly beneficial results. A 2011 meta-analysis proposed that variations in zinc dosage may explain a substantial part of the heterogeneity between published trials. The author concluded that three high dose zinc acetate lozenge trials showed a 42% reduction in the duration of colds but that, in contrast, trials of low-dose treatments gave negative results.
Potential biological mechanisms
Evidence-based medicine focuses primarily on the effect of interventions on clinically relevant outcomes in controlled trials. Nevertheless, the potential biological mechanisms are of interest. In laboratory studies, zinc inhibited the replication of respiratory viruses and enhanced the effect of interferon. Non-immune mechanisms have also been proposed to explain the effect of zinc lozenges on the common cold.
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