A cochrane review suggested that taking zinc within onset of cold symptoms is effective at lingering the common cold.
What's the optimal dosage?
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As the study says:
Zinc administered within 24 hours of onset of symptoms reduces the duration of common cold symptoms in healthy people but some caution is needed due to the heterogeneity of the data. As the zinc lozenges formulation has been widely studied and there is a significant reduction in the duration of cold at a dose of ≥ 75 mg/day
Superloading zinc by taking up to 100mg zinc a day is confirmed to be safe in the short term (2-4 months), but because this dose is higher than the 40mg Tolerable Upper Limit (TUL) of zinc, prolonged superloading is not advised.
Supplementation of 45mg elemental zinc for a period of 12 months in elderly persons who had low baseline zinc levels, the infection rate seen with supplementation (29%) was lower than placebo (88%) although upper respiratory tract infection rates trended to be less common (50% reduction) but this failed to reach significance.
The 2013 Cochrane study was trashed by one reviewer and got taken down. There is no significant evidence that it reduces the common cold symptoms in adults if taken as one is starting.