One problem with zinc is that dosing isn't standardized and people use it in stupid ways, cf. nasal sprays. High concentrations of heavy (-er than calcium) met

One problem with zinc is that dosing isn't standardized and people use it in stupid ways, cf. nasal sprays. High concentrations of heavy (-er than calcium) metal ions are of course neurotoxic; putting high concentrations of zinc in direct contact with olfactory tissue is to be avoided. Many supplements contain 50 mg of zinc or more, and worse, people take multiple pills. However, studies show similar effects using only 12.5 mg of zinc: https://www.ncbi.nlm.nih.gov/pubmed/11073753?dopt=Abstract https://www.ncbi.nlm.nih.gov/pubmed/11073753?dopt=Abstract Studies using much higher amounts of zinc do not show larger effects than this. The duration reduction in the above study was 28% while studies using 200 mg found a reduction of 35%: https://journals.sagepub.com/doi/full/10.1177/2054270417694291 https://journals.sagepub.com/doi/full/10.1177/20542704176942... Simply put, it does not make sense to multiply the dose by a factor of sixteen in order to improve effectiveness by 20%. Yet I don't know exactly what GP is talking about. There are loads of studies about zinc for the common cold and there are loads of pills on store shelves containing zinc and claiming to treat the common cold. At least here in the United States, that is. People don't take it because of the side effects and the stupid dose forms available (although the situation seems to be slowly improving).