The herbal remedy echinacea, taken by millions to cure colds, does not work, US research has shown.

It found that cold sufferers taking echinacea were actually ill for longer than those who took dummy pills.

Many believe echinacea, which can be made into capsules, extracts, tinctures and tea, boosts the immune system.

The remedy is widely used to treat and prevent colds, but previous studies looking at its effectiveness have been inconclusive.

Echinacea facts

Also known as the coneflower, it grows on the US prairies There are nine species, but only three are used in herbal medicine Native Americans have used it for centuries to treat a range of conditions

In the US study, 150 students who had recently developed colds were given echinacea or dummy pills - but were not told which group they were in.

A mixture of Echinacea purpurea herb and root and Echinacea angustifolia root was used in the trial.

Participants evaluated symptoms including cough, runny nose and sore throats daily.

No difference in severity of symptoms was found between those who took echinacea and those who did not.

On average, the colds of echinacea-takers lasted slightly longer - 6.27 days compared to 5.75 days amongst those on the dummy pills.

'Not the last word'

The research team, from the University of Wisconsin, wrote in the journal Annals of Internal Medicine: "We consider the results of this trial to be negative."

But they admitted that could be due to the specific preparation of echinacea they used, which could be different to other versions of the herbal remedy.

Any expectation of benefit is based on faith rather than science Professor Ronald Turner, University of Virginia

The team, headed by Dr Bruce Barrett, added: "Our results do not support a benefit of echinacea in the treatment of common cold symptoms."

But he said the trial should not be "the last word" on echinacea, and further research was needed.

In an editorial in the same journal, Professor Ronald Turner of the University of Virginia, said people buying echinacea were wasting their money.

"Nobody knows what echinacea's active ingredients are.

"Any expectation of benefit is based on faith rather than science.

"It's entirely possible to buy the same product over and over again and it actually has different levels of material in it."

Threat

But Trudy Norris, of the UK's National Institute of Medical Herbalists told BBC News Online said echinacea was a very popular herbal remedy.

"A lot of people do self-prescribe, and I've never had a bad report for echinacea.

"People should continue to take it if they feel it's beneficial to them. And I have to say that I believe echinacea is beneficial."

In the UK, the herbal remedy market is worth around �126m a year. The cost of echinacea ranges between �6 and �10 for 60 capsules.

A proposed European Union directive would require all herbal medicines to be registered.

Packs would have to include a full list of ingredients and would have to prove they are not a threat to public health.