Zinc supplements in high enough doses may shorten the duration of common colds by more than 40 per cent, a study has found.
Scientists analysed the combined results of 13 trials testing the ability of zinc lozenges to fight off colds.
Three studies showed that daily doses of zinc acetate higher than 75 milligrams shortened colds by an average 42 per cent.
Five others using other types of zinc salt at doses greater than 75mg produced a 20 per cent reduction in cold duration.
But five studies of zinc doses lower than 75mg showed no benefit at all.
There was no evidence that the lozenges, which dissolve slowly in the mouth, caused any long-term harm.
Zinc is important to the immune system and deficiency is known to increase the risk of infection.
The idea that zinc lozenges might work against colds stems from an accidental observation in the early 1980s.
Doctors saw that the cold of a three-year-old girl with leukaemia vanished when she dissolved a therapeutic zinc tablet in her mouth instead of swallowing it.
Since then a number of studies have looked at the effects of zinc on cold symptoms, with inconclusive results. While some have suggested the mineral can fight colds, others have indicated it is no help.
The new findings, published in the Open Respiratory Medicine Journal, point to the poor evidence being due to different doses.
The researchers, led by Harri Hemila, from the University of Helskinki in Finland, wrote:
“A large part of the divergence can be explained by the variation in the total daily dose of zinc that the person obtained from the lozenges. Many trials with daily zinc doses of over 75mg have found significant reduction in the duration of colds.
“Zinc lozenges have caused adverse effects, such as bad taste, but there is no evidence that they would cause long term harm.
“Since a large proportion of trial participants have remained without adverse effects, zinc lozenges might be useful for them as a treatment option for the common cold.
“More research is needed on zinc lozenges to find optimal lozenge compositions and treatment strategies.”