A visit by Which? to key supermarkets, chemists and smaller health shops in London in October found numerous examples of unsubstantiated claims on supplements, with the worst culprits those which claimed to maintain healthy bones and joints, chief executive Peter Vicary-Smith said.
"Researchers also found high-strength supplement products containing vitamin B6 and beta-carotene on sale, without the recommended warnings that taking too much of them could be harmful," he added.
In addition to visiting retail outlets, Which? conducted an online survey of 1,263 supplement takers across the UK.
"A third didn't realise that taking too much of some supplements could damage your health," Mr Vicary-Smith said.
He called on the European Commission to address the issue.
"We're concerned that people are being taken for a ride, needlessly paying a premium for many products on the basis of health claims that haven't been backed up by scientific evidence," he said.
"We want to see the European Commission release a list of accepted and rejected claims as soon as possible, so consumers won't continue to be bamboozled by health claims they can't trust."
The study comes just weeks after experts warned that supplements taken by millions of arthritis sufferers worldwide do not work.
Glucosamine and chondroitin are frequently used to try to alleviate pain from osteoarthritis, which mostly occurs in the knees, hips and small joints.
The National Institute for Clinical Excellence (Nice) said neither supplement – in any form – is backed by evidence.
The study said global sales of glucosamine supplements reached almost 1.3bn in 2008.