Many herbal medicines are being sold with dodgy labelling and fail to comply with regulations, a new study has found.

Researchers at the University of Adelaide discovered that almost 20 per cent of the herbal remedies surveyed - including vitamins, minerals and fish oils available at supermarkets and pharmacies - are not registered with the Therapeutic Goods Administration, despite it being a legal condition for their sale.

And almost 60 per cent had ingredients that did not match what is listed on the bottle or pack.

''We are talking about significant differences between the stated concentrations and what the manufacturers have had approved with the TGA,'' said lead author and senior lecturer in pharmacology at University of Adelaide, Ian Musgrave.

He said any changes to popular therapies such as ginkgo, dandelion and St John's wort can have serious drug interactions.