Zinc supplements have been confirmed in a large international study as an effective treatment of the common cold, shortening symptoms by nearly one day.

The review by the Cochrane Collaboration, published today, has also found that zinc supplements can protect against catching colds.

The review updates earlier Cochrane research and includes data from 15 trials involving 1360 people using zinc tablets, syrup or lozenges.

"This review strengthens the evidence for zinc as a treatment for the common cold," said the lead researcher, Meenu Singh of the Post Graduate Institute of Medical Education and Research in Chandigarh, India.

"However, at the moment, it is still difficult to make a general recommendation, because we do not know very much about the optimum dose, formulation or length of treatment."

The common cold is estimated internationally to account for 40 per cent of time taken off work and millions of days off school for children.

In the studies included in the latest Cochrane review, the average duration of cold symptoms for those given a placebo was 5.1 to 8.5 days.

It was just under one day less for those who started taking zinc soon after symptoms started.

People who took zinc for at least five months as a preventive measure had 38 per cent fewer colds than those in the placebo groups.

The researchers say in their paper there is no proven treatment for the common cold.

"However, a medication that is even partially effective ... could markedly reduce morbidity [illness] and economic losses due to this illness."

They also said that as no studies had been done regarding people for whom colds could cause particular problems, such as asthma, "the use of zinc currently cannot be recommended for them".

Otago University respiratory physician Professor Robin Taylor yesterday described the review as moderately strong evidence in favour of zinc as a cold treatment and preventive measure.