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.

''In some cases, manufacturers are adding in or substituting herbs with entirely different active ingredients which can be highly toxic and can lead to adverse affects.''

The study, to be published in the Journal of Forensic and Legal Medicine next month, examined 121 herbal products, including 29 claiming to treat arthritis, 33 for cold and flu, 19 for gastrointestinal disorders, 30 for stress and 10 for premenstrual syndrome. Only 15 products had ingredients that were consistent with their TGA listing and product packaging.

Products in the gastrointestinal and arthritis categories - which include slippery elm bark, senna leaf and aloe - had the highest level of non-compliance.

''If the herbal medicine manufacturers cannot follow regulation and can't go through the re-registration when they make major changes, then it throws the entire herbal medicine manufacturing process into doubt,'' Dr Musgrave said.

He said there were more than 10,000 herbal medicines registered or listed on the TGA, with many people unaware they could have serious interactions with conventional medication.

''Dandelion is often used as a diuretic but in combination with blood pressure medication it can be very harmful.''

Consumers should always look for an ''AUST L'' or ''AUST R'' number printed on the front of complementary medicine packaging, Dr Musgrave said, as this showed it complied with TGA standards.