An analysis of 17 patient trials comparing oral zinc preparations to placebo found that sucking on the lozenges appeared to shorten the duration of the common cold by about two days.
But lead author Dr. Michelle Science, an infectious disease specialist at Toronto's Hospital for Sick Children, said the review of the trials involving a total of more than 2,100 patients did not show that using zinc alleviated the severity of symptoms.
"Overall, what we found was that oral zinc taken at the start of a cold, or at the onset of a cold, reduced the average amount of time that a person will have symptoms, such as runny nose or congestion," Science said Monday.
"But the reduction was relatively minor, so less than two days on average was what we found. If the average cold lasts seven to 10 days, then take off perhaps on average a day and a half of symptoms."
There was some evidence that adults taking zinc were less likely to have symptoms after one week, although there was no difference in symptoms between the two groups at three days.
More than 100 viruses can cause the "common cold," which can leave people with a sore throat and trying to breathe through a runny nose, while sneezing, coughing and battling headache and fatigue. Fever is more common in children than in adults. Symptoms typically last a week to 10 days, but can stretch to two weeks.
Zinc did not appear to help children get over a cold sooner. The few clinical trials included in the meta-analysis — in which conclusions are drawn after pooling data from several studies — found kids did not seem to fare better with zinc than with a dummy preparation.
There could be several explanations for the difference found in adults and children, Science said. For one, most children in the studies got the zinc in a syrup twice a day, resulting in a proportionately lower daily dose than that taken by adults.
"One of the proposed mechanisms is that zinc has local action. So you can imagine that a syrup swallowed may have less of an effect than a lozenge dissolved in the mouth for a more prolonged period," she said, noting that adults generally had a lozenge every two hours, up to eight times a day.
As well, the studies were conducted at various times of the year, when different viruses that cause cold symptoms are circulating and causing infection.
The studies involving children took place during winters in the northern hemisphere, so some of the kids could have been infected with respiratory syncytial virus, which causes upper respiratory infections much like a cold.
Adults were studied throughout the year, when rhinoviruses — which cause 50 per cent of colds — were more likely being transmitted. Zinc has been shown in previous laboratory studies to inhibit rhinoviruses.
The study, published Monday in the Canadian Medical Association Journal, found that not all zinc formulations appear to be equal. Zinc acetate seemed to have a more pronounced effect than either zinc gluconate or zinc sulphate.
"The whole idea of zinc being used for colds is really an old one," said Dr. Samira Mubareka, a microbiologist and clinician at Sunnybrook Health Sciences Centre in Toronto. "I think back in the '70s, they were doing experiments with zinc and rhinovirus.
"So I think it's been on people's radars for a long time. People have tried to put it forward because it would be a ... simple, inexpensive supplement to actually reduce the cold."
However, the numbers of patients in individual trials have been too small to come to robust conclusions on their own. And while the meta-analysis has more strength in its numbers, the researchers say their findings still can't provide the last word on zinc for colds.
"So I think at the end of the day, if someone were to ask me about this study, I would say it does show that at least there is some potential for zinc," said Mubareka. "And it does support going further with it, instead of saying, 'Oh well, look, there's really no difference, forget about it.'
"I certainly wouldn't start taking it myself or giving it to my kids or family members, and I wouldn't start telling patients to take zinc themselves." But she said there is enough potential in zinc that it warrants further research.
Still, using zinc does have some downsides: the metallic element leaves a bad taste in the mouth and can cause ongoing nausea, which may outweigh its upside of a modest shortening of cold symptoms, Science said.
"Although it's possible that oral zinc could impact the symptoms of the common cold, I would say there's not enough evidence to recommend its use in children, and only a weak rationale for otherwise healthy adults," she said.
"Further large, high-quality trials are needed before more definitive recommendations can be made."