They are the expensive, pocket-sized drinks that promise to improve your well-being. However, half of the probiotic supplements that claim to boost "intestinal health" do not do what they say on the packaging.

Sales of probiotics have soared but experts said yesterday that most of the 50 brands marketed in the UK contain far fewer of the "friendly bacteria" than claimed, or none at all. Some even contain harmful bacteria.

Professor Glen Gibson, an expert on probiotics at Reading University, said: "As a rule of thumb you can trust the big manufacturers - they have their quality control right.But consumers should avoid brands that they have never heard of."

Probiotic drinks, which should contain at least 10 million "healthy" bacteria per daily dose from the bifido or lactobacillus groups, are claimed to improve digestion and prevent bowel problems. Professor Christine Edwards, head of human nutrition at the University of Glasgow, said consumers should check the packaging to see that products carried the name of the bacterium, the strain and the number of bacteria.