Researchers led by Dr Klaus Linde from the Centre for Complementary Medicine Research in Munich examined 16 trials comparing single Echinacea preparations with a control group that had been carried out in recent years.
The majority had "reasonable to good methodological quality", they said in the paper, published in the Cochrane Database of Systematic Reviews 2006 (issue 1).
But the trials tested different species and different extracts - from the root, leaves or both - as well as different manufacturing methods for producing the Echinacea preparations.
The type of extract appears to have a significant impact on whether the herbal can fight off colds.
Nine of the trials found that extracts from the aerial parts of Echinacea purpurea could reduce the symptoms of colds in adults if started early enough, although these results were not fully consistent, said the authors.
They found positive effects from alcoholic extracts and pressed juice preparations of the aerial parts of the plants, but they were not able to find benefits from other preparations.
The conclusions demonstrate the importance of labelling Echinacea products with information about the extract used, and communicating the differences to consumers.
"Consumers and clinicians need to realise that Echinacea products differ greatly in the species of plant utilised, the parts of the plant used, way they have been prepared and in the active agents they contain," said Dr Linde.
He added that the "overwhelming majority" have not been tested in clinical trials.
Two further trials that looked at whether the extract could prevent people catching colds showed no effect.
"Other preparations of Echinacea might have preventative effects, but the effect has not been shown in independently replicated, rigorous randomised trials," said Dr Linde.
Echinacea extracts contain a mixture of different active components, each of which may act on their own, or the effect may only be achieved as they act together.
"If this synergistic action does occur, then the exact composition of the extract will greatly affect its performance," said Dr Linde.
Given the widespread use of Echinacea products, further research is clearly desirable, added the authors, and this should be carried out on chemically well-defined preparations so that comparisons can be made.
Sales of echinacea products in the United States generated $173.2 million in 2004 while in Germany they reached $107.4 million. But efficacy in prevention or treatment of colds will be key to the market's continued growth.