Taking the herbal remedy echinacea can more than halve the risk of catching a common cold, US researchers say.
The flower, stem and root of echinacea is used in products
They found it decreased the odds of developing a cold by 58% and the duration of colds by a day-and-a-half.
The results in The Lancet Infectious Diseases conflict with other studies that show no beneficial effect.
Experts believe echinacea, a collection of nine related plant species indigenous to North America, may work by boosting the body's immune system.
Researchers, led by Dr Craig Coleman from the University of Connecticut School of Pharmacy, combined the results of 14 different studies on Echinacea's anti-cold properties.
In one of the 14 studies the researchers reviewed, echinacea was taken alongside vitamin C. This combination reduced cold incidence by 86%.
When echinacea was used alone it reduced cold incidence by 65%.
Even when patients were directly inoculated with a rhinovirus - the most common cold-causing virus - echinacea reduced cold incidence by 35%.
The researchers' report said: "With over 200 viruses capable of causing the common cold, echinacea could have modest effect against rhinovirus but marked effects against other viruses."
They found that more than 800 products containing echinacea were available, and that differing parts of the plant - flower, stem and root - were used in different products.
They said more work was needed to check the safety of these different formulations.
Professor Ron Cutler, of the University of East London, said: "The true benefits, and more importantly, how the agents work remains unclear and further better-controlled actual clinical trials still have to be carried out.
THE COMMON COLD
Rhinoviruses are responsible for about half of all common colds in children and adults
School children usually catch between seven and 10 colds a year, and adults two to five
Common colds and flu can be transmitted by hands and contact with commonly-touched surfaces
"Echinacea may reduce the duration of illness and decreases the severity of cough, headache, and nasal congestion. "
He said people with impaired immune function might benefit from taking echinacea during the winter months to prevent colds and flu, but that healthy people did not require long-term preventative use.
"There has also been the suggestion in the past that continuous treatment with echinacea is not recommended - the benefits may only be effective for one or two weeks and after taking the agent for this time people should stop and give the immune system a week without the agent."
Professor Ronald Eccles, director of the Common Cold Centre at the University of Cardiff, said the work was "a significant step in our battle against the common cold".
"Harnessing the power of our own immune system to fight common infections with herbal medicines such as echinacea is now given more validity with this interesting scientific evaluation of past clinical trials," he added.