Researchers who visited supermarkets, chemists and smaller health shops in London in October found numerous examples of unsubstantiated claims on supplements.
Which? chief executive Peter Vicary-Smith said the worst culprits were those that claimed to maintain healthy bones and joints.
Claims about key ingredients including glucosamine and long chain omega-3 fatty acids have all been turned down by the European Food Safety Authority.
However, until the regulations have been fully implemented they will still appear on bottles.
“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,” Vicary-Smith 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,” Smith said.