Zinc was recognized as an element in 1509 and as an essential mineral much later. In 1961, zinc deficiency was linked with hypogonadism. Zinc is included in almost all over-the-counter daily vitamins and mineral supplements, typically in the form of zinc oxide, zinc acetate, and zinc gluconate. Zinc is absorbed through the small bowel with an efficiency of 20%-40%. It is the second most important metal in the body after iron and is present in virtually 100% of proteins.
Zinc inhibits viral replication. Because of this, it has been investigated as a way to decrease the duration of symptoms from the common cold. With some evidence suggesting that it works, we know little about the right dose for zinc to exert its magical effects.
In a recently published systematic review, Singh and Das updated a previous Cochrane systematic review and, once again, evaluated the efficacy of zinc in reducing the incidence, severity, and duration of common cold symptoms (Cochrane Database Syst. Rev. 2013;6:CD001364). Studies were included if they were randomized, double-blind, placebo-controlled trials using zinc for at least 5 days for treatment or 5 months for prevention of the common cold.
Sixteen therapeutic trails involving a total of 1,387 people, and two preventive trials with 394 participants were included in the meta-analysis. Zinc came in the form of syrup, lozenges, or tablets. Zinc was associated with statistically significant reductions in the duration but not the severity of symptoms. The mean difference in reduction duration was 1 day (95% confidence interval, –1.72 to –0.34). After 7 days of treatment, significantly fewer subjects had symptoms. Zinc was associated with a reduced incidence of colds, absences from school, and receipt of antibiotics. Bad taste and nausea were significantly higher in patients treated with zinc. The authors suggested that there is a significant reduction in the duration of cold symptoms at a dose of at least 75 mg/day in the lozenge form.
Inhaled zinc can cause permanent anosmia, and so this delivery route was not investigated. Lozenges may be the best bet, since we know the daily dose should be at least 75 mg for treatment. For patients interested in using zinc for prevention, no clear dosage recommendations can be made. Megadose supplementation or high zinc intake has been associated with abdominal pain, diarrhea, nausea, and vomiting. Zinc may interfere with copper absorption, and high zinc intake (greater than 150 mg/day) can lead to copper deficiency and should be avoided.
Dr. Ebbert is professor of medicine and a general internist at the Mayo Clinic in Rochester, Minn., and a diplomate of the American Board of Addiction Medicine. The opinions expressed are those of the author. He reports no conflicts of interest.