Zinc is needed by hundreds of enzymes and by the immune system, and true deficiency, common in parts of the world and rare in Western diets, causes growth failure, poor wound healing and susceptibility to infection. As a supplement in well-fed countries it is sold for immunity, skin, testosterone and above all the common cold.

The cold story is a lesson in how the same trials can be read two ways. Lozenges that dissolve slowly in the mouth and release zinc ions are proposed to act locally on the virus in the throat. Some trials of high-dose acetate lozenges found striking reductions in cold duration. Others, including a recent well-run one, found none. The reviews split along the same line.

Zinc is a structural and catalytic component of enzymes across metabolism, and immune cells need it to develop and signal, which is why deficiency impairs immunity. The lozenge hypothesis is different and local: zinc ions released in the throat are proposed to interfere with rhinovirus replication and with the inflammatory signaling that produces symptoms. That mechanism depends on the salt, since acetate releases free ions and gluconate releases fewer, and on the lozenge dissolving slowly rather than being swallowed.

The case for. A 2016 individual-patient meta-analysis of three trials of zinc acetate lozenges, 199 patients, found colds about 2.7 to 2.9 days shorter against an average of seven, with the effect consistent across age, sex, allergy status and smoking. A 2017 reanalysis of the same trials found patients on zinc recovered about three times faster, with 70% recovered by day five against 27% on placebo. A 2015 analysis found the shortening spread across nasal, throat and systemic symptoms.

The case against. A 2020 randomized trial of a commercially available zinc acetate lozenge in 253 Finnish adults found no difference in recovery rate between zinc and placebo over ten days, and the zinc group recovered more slowly in the two days after stopping. The 2024 Cochrane review of 34 studies in 8,526 people concluded the evidence on whether zinc prevents or shortens colds is uncertain, with adverse effects such as bad taste more common on zinc. A 2020 systematic review of micronutrients reached a middle verdict: zinc given alone may shorten colds by about two days, with low certainty.

Everything else. Claims about testosterone, skin and general immunity in people who are not deficient rest on small trials and on extrapolation from deficiency studies.

Short courses of zinc lozenges cause bad taste, nausea and mouth irritation in a large minority of users; in the 2020 trial, two thirds of the zinc group reported adverse effects. Intranasal zinc products caused permanent loss of smell and were withdrawn in the United States in 2009; that was a route problem, not an oral one, and it is why no intranasal zinc appears here.

Longer-term high intake interferes with copper absorption and can cause copper deficiency with anemia and nerve damage. Nutrition authorities publish an upper limit for that reason. Zinc also reduces the absorption of some antibiotics if taken at the same time.

Zinc is sold as a dietary supplement and in lozenge form as an over-the-counter cold remedy in most markets, with the usual absence of claim review. In the United States, cold lozenges are marketed as supplements or as homeopathic products rather than as approved drugs. Nutrition authorities publish a reference intake and an upper limit, which this site links to. The withdrawn intranasal products are the one regulatory action worth knowing about.