For years, researchers have remained dubious about zinc as a remedy for the common cold.
New evidence, however, suggests that oral zinc may shorten the duration of cold symptoms – if patients can stomach the treatment.
A study published in the Canadian Medical Association Journal found that nausea was a common reaction to oral zinc preparations.
Of 2,121 participants in 17 randomized controlled trials, about 17 per cent of people taking zinc remedies experienced nausea, while about 35 per cent said zinc left a bad taste in the mouth.
On the plus side, zinc lozenges appeared to shave 1.6 days off the usual seven-day cold. But oral zinc did not reduce the severity of cold symptoms, such as sneezing, coughing and congestion, says Michelle Science, an infectious diseases physician at the Hospital for Sick Children in Toronto, who co-authored the study with researchers at McMaster University in Hamilton.
Although the study suggests that oral zinc may reduce the duration of the common cold, she says, "there is not enough evidence to recommend its use in children and only a weak rationale [for use]in otherwise healthy adults."
The treatment regimen may be unappealing to cold sufferers, she adds. At the onset of cold symptoms, participants in most of the trials sucked on a zinc lozenge every two hours when awake. "Some people may consider it to be onerous," Dr. Science points out.
The remedies in the trials consisted of zinc alone and not in combination with other minerals. The trials included three different formulations: zinc acetate, zinc gluconate and zinc sulphate.
The researchers found that zinc acetate, in particular, and treatment regimens that resulted in higher total daily doses of ionic zinc – the form of zinc released in the body – seemed to have the most beneficial effects. But the studies varied greatly in their methods and results, says Dr. Science, who emphasizes that further research is needed.
"At this point it's difficult to make a firm recommendation on the dose or formulation that's optimal for use," she says.
An earlier study by Harri Hemila of the University of Helsinki in Finland found that some forms of zinc are not readily available to the body, whereas zinc acetate appeared to be a highly effective cold remedy.
In Dr. Hemila's analysis of 13 placebo-controlled studies, three trials showed that a daily dose of zinc acetate greater than 75 milligrams shortened the typical seven-day cold by four days.
Many supplements sold in Canadian pharmacies do not contain zinc acetate, however. And cold sufferers need to weigh the discomfort of coughing and sneezing against the risk of nausea and the bitter experience of sucking on zinc lozenges all day.
"The decision to take zinc should take into consideration the questionable benefits balanced with these potential adverse effects," Dr. Science says.