That said, the supplements didn't lessen the severity of these symptoms in adults, and had no effects on children, who can catch as many as eight to 10 colds per year. Zinc lozenges or liquids also have their share of side effects, including bad taste and nausea.
"Although oral zinc can impact the duration of [the] common cold in adults, there is not enough evidence to recommend its use in children, and only a weak rationale for its use in adults," says researcher Michelle Science, MD. She is an infectious disease specialist at the Hospital for Sick Children in Toronto, Ontario, Canada.
Exactly how zinc shortens the duration of the common cold is unknown, but it may stop the virus from replicating. The new review included 17 studies comprising a total of 2,121 participants between 1 and 65 years old. Overall, zinc did shorten the duration of the common cold, but only in adults. The higher doses worked better than lower doses.
The findings appear in the Canadian Medical Association Journal.
It is possible that the review did not contain enough studies of children and/or cold symptom severity to draw a firm conclusion.
This is not the final word on zinc. "We need further research and high-quality trials to optimize and maximize the daily doses," Science says.
Not All Zinc Supplements Are Created Equally
As an internist at Lenox Hill Hospital in New York City, Robert Graham, MD, is often asked if anything can be done to shorten the duration of a cold and/or lessen the severity of its symptoms.
And in many cases, zinc supplements can fit this bill.
"Choose a quality zinc product with at least a couple of seals of approvals, including the Good Manufacturing Practices (GMP) seal of approval," he says. This is a way to make sure that the product contains what is stated on its label.
There are still many questions about which form and dose of zinc is best and for whom. "These are the questions we need to answer next," he says.
Zinc researcher Ananda Prasad, MD, PhD, takes issue with the new study's conclusions. He is a distinguished professor of medicine at Wayne State University in Detroit, Mich.
For starters, he says, many of the zinc formulations included in the review were likely inactive. "The chemistry is important," he says. "The chemical composition has to be right so zinc is released, and it must be started within 24 hours of cold onset to be effective."
He suggests only choosing zinc acetate or zinc gluconate lozenges.