Studies generally support the use of zinc for the common cold and similar upper-respiratory tract infections. The evidence in zinc's favor is substantially better than that for any other purported cold remedy, as per numerous trials and meta-analyses. [1, 2, 3, 4]
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Zinc deficiency is also a risk factor for pneumonia in the elderly, which suggests that zinc supplementation may result in reduced risk and mortality from pneumonia. 
Zinc’s potential utility as a prophylactic measure against upper-respiratory tract infections is limited by the fact that it is only effective in a very narrow range of formulations, for zinc’s most plausible mechanism of action requires contact between unbound, ionic zinc and infected cells. This has several implications:
- For upper respiratory tract infections, capsules will prove poorly-effective or entirely ineffective. Lozenges, instead, are the ideal dosage form.
- Very few common zinc compounds release sufficient amounts of ionic zinc at physiologic pH; zinc acetate and zinc gluconate are, at the present time, the only viable sources of ionic zinc. Of the two, zinc acetate is considerably more effective.
- Zinc acetate can react with other ingredients in the lozenge, and in saliva, which typically results in stable reaction products that do not release ionic zinc at physiological pH. Therefore zinc acetate lozenges must not contain excipients such as tartaric acid, stearate, magnesium, ascorbic acid, among many others. They must not contain Vitamin C. They should not contain plant extracts.
Although zinc is a common supplement, effective zinc lozenges that have been properly formulated are exceedingly rare.
1.Science, M.; Johnstone, J.; Roth, D. E.; Guyatt, G.; Loeb, M.; Zinc for the treatment of the common cold: A systematic review and meta-analysis of randomized controlled trials. Canadian Medical Association Journal. (2012). 184 (10): E551–61.
2.Eby G.A.; Zinc lozenges as cure for the common cold--a review and hypothesis. Med Hypotheses. 2010;74(3):482-92.
3.Hemilä H, Petrus EJ, Fitzgerald JT, Prasad A. Zinc acetate lozenges for treating the common cold: an individual patient data meta-analysis. Br J Clin Pharmacol. 2016;82(5):1393-1398.
4.Hemilä H, Chalker E. The effectiveness of high dose zinc acetate lozenges on various common cold symptoms: a meta-analysis. BMC Fam Pract. 2015;16:24.
5.Meydani SN, Barnett JB, Dallal GE, et al. Serum zinc and pneumonia in nursing home elderly. Am J Clin Nutr. 2007;86(4):1167-73.