Zinc lozenges are available in a variety of formulations to treat the common cold. They remain somewhat controversial. The main questions are: Do they work? If so, does zinc boost the immune system or attack the viruses directly? What is the effective dose? And is it safe to consume this much zinc?
It's important to know the mechanism(s) of action because if zinc kills the virus directly, then it makes sense to use zinc lozenges and/or nasal sprays. If immune support is the only effect, then a simple pill might be just as effective.
Safety was demonstrated in a clinical trial of Cold-Eeze in people 60 to 91 years old. The study participants took one of these lozenges or a placebo every 3 to 4 hours for 6 days. The researchers assessed both groups for health effects at baseline, 7 days, and 14 days. They found Cold-Eeze to be as safe as placebo.
For effectiveness, different researchers report different results. Part of this variation comes from the difference in dosage and lozenge formulation: zinc acetate, zinc gluconate, zinc orotate, and other forms have been tested.
One of the most promising clinical trials I found was conducted at Wayne State University. The researchers prepared zinc lozenges and placebos with the same look and taste. Tests on healthy volunteers confirmed that the placebos were indistinguishable from the test lozenges, which contained 42.96 mg of zinc acetate dihydrate (12.8 mg of zinc). Fifty volunteers with colds were randomized to zinc or placebo. Volunteers dissolved one lozenge in their mouths every 2 to 4 hours while awake, as long as they had cold symptoms. The researchers measured levels of three proinflammatory cytokines to see how the zinc affected the immune system. Volunteers reported their cold symptoms on a daily log.
Zinc was highly successful in this trial: the average duration of cold symptoms was 4.5 days in the zinc group, and 8.1 days in the placebo group. For individual symptoms, zinc had the most effect on cough: average 3.1 days in the zinc group, and 6.3 days in the placebo group.
The mechanism for zinc's benefits was not established.