Vitamin C's reputation for fighting the common cold may not be justified, say researchers.

They say they checked the best studies done on the topic in the last 65 years. Their findings appear in Public Library of Science Medicine.

Researchers working on the review included Robert Douglas, MD, of the Australian National University. They found hints of possible benefits in some conditions.

Those areas should be studied further, say Douglas and colleagues. But overall, they say they didn't find proof that vitamin C thwarts colds.

Read WebMD's "Understanding Common Cold -- Prevention"

Cold Prevention Questioned

The review included 55 studies.

Vitamin C did not prevent colds in people taking up to 2 grams daily, say the researchers.

However, there were exceptions. In six studies, the number of colds was halved with vitamin C.

But those studies involved rare, extreme conditions. The number of colds was only cut in marathon runners, skiers, and soldiers exposed to cold temperatures and/or physical stress.

Those benefits should be treated with "great caution" and probed further, say the researchers.

Read WebMD's "Tips for Getting Vitamin C"

Shorter Colds?

In the cold-prevention studies, vitamin C didn't keep colds at bay for most people. But it may have slightly shortened colds for kids and adults.

Children regularly taking vitamin C had cold symptoms for 14 percent fewer days. For adults, days with cold symptoms fell 8 percent with regular vitamin C use, say the researchers.

The vitamin's cold-shortening pattern was consistent. But it may have "questionable" significance in the real world, say the researchers.

Read WebMD's "Vitamin C: How Much Do You Really Need?"

Treating Colds With Vitamin C

Does it help to start taking vitamin C at the first sign of a cold? No, say most of the studies exploring that question.

Seven studies covered the topic. Only one showed that colds were shorter with vitamin C taken at the onset of symptoms.

In that large study, people took a vitamin C megadose -- 8 grams - only on a cold's first day.

The results are "tantalizing," say Douglas and colleagues.

They call for studies on the value of high-dose vitamin C therapy for colds, especially in children.

By Miranda Hitti, reviewed by Brunilda Nazario, MD

SOURCES: Douglas, R. Public Library of Science Medicine, June 2005; vol 2: pp 132-133. News release, Public Library of Science.