There's been a lot of talk about taking zinc for colds ever since a 1984 study showed that zinc supplements kept people from getting as sick. Since then, research has turned up mixed results about zinc and colds.

Recently an analysis of several studies showed that zinc lozenges or syrup reduced the length of a cold by one day, especially when taken within 24 hours of the first signs and symptoms of a cold.

Most colds are caused by a type of virus called rhinovirus, which thrives and multiplies in the nasal passages and throat (upper respiratory system). Zinc may work by preventing the rhinovirus from multiplying. It may also stop the rhinovirus from lodging in the mucous membranes of the throat and nose.

Zinc may be more effective when taken in lozenge or syrup form, which allows the substance to stay in the throat and come in contact with the rhinovirus.

But the recent analysis stopped short of recommending zinc. None of the studies analyzed had enough participants to meet a high standard of proof. Also, the studies used different zinc dosages and preparations (lozenges or syrup) for different lengths of time. As a result, it's not clear what the effective dose and treatment schedule would be.

Zinc — especially in lozenge form — also has side effects, including nausea or a bad taste in the mouth. Many people who used zinc nasal sprays suffered a permanent loss of smell. For this reason, Mayo Clinic doctors caution against using such sprays.

In addition, large amounts of zinc are toxic and can cause copper deficiency, anemia and damage to the nervous system.

For now, the safest course is to talk to your doctor before considering the use of zinc to prevent or reduce the length of colds.