A new Canadian review concludes that taking certain zinc products may help cut the duration of common cold symptoms such as a runny nose in an adult, but there remains "weak rationale" for recommending it as a treatment.
Researchers looked at 17 randomized controlled trials with 2,121 participants aged one to 65 to assess the safety of zinc lozenges and syrup for treating the common cold.
Colds are benign for most people, but can lead to substantial illness resulting in workplace absenteeism and lost productivity, said Dr. Michelle Science of the Hospital for Sick Children in Toronto and her team.
"We found moderate quality of evidence to suggest that orally administered zinc reduces the duration of symptoms of the common cold," Science and her co-authors from McMaster University in Hamilton concluded.
"Until further evidence becomes available, there is only a weak rationale for physicians to recommend zinc for the treatment of the common cold. The questionable benefits must be balanced against the potential adverse events."
Study subjects taking zinc products were more likely to experience bad taste and nausea compared to participants on placebos.
The investigators found weak evidence that people on the lozenges or cold syrup were less likely to have symptoms after one week, although there was no difference in symptoms between the two groups at three days.
Previous studies pointed to conflicting effects of zinc in reducing the severity and duration of cold symptoms.
Science’s team found no significant effect among children. Possible reasons for the finding include:
- Age-related differences in inflammatory responses.
- Exposure to different cold viruses.
- Lower dosing.
- Use of syrups instead of lozenges in kids who took the treatments less often.
"One of the proposed mechanisms is that zinc has local action. 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," Science said, noting that adults generally had a lozenge every two hours, up to eight times a day.
Not all zinc formulations may be created equal. Zinc acetate seemed to have a more pronounced effect than either zinc gluconate or zinc sulphate.
The researchers said the adverse events call for more study, saying the safety of taking zinc is essential for what’s generally a mild illness.
There was no dedicated funding for the study.