Australia’s drug regulator is reviewing its guidelines for fish oil supplements after a study found consumers were being deceived into buying potentially contaminated products containing much lower levels of omega-3 fatty acids than advertised.

Of 32 brands of fish oil supplements examined, only three contained quantities of omega-3 fatty acids at or near the levels stated on the label, the research published in the Nature journal, Scientific Reports, found.

The research also found markers that the fats had oxidised – or were going off – exceeded international recommendations in 92% of the supplements studied.

While there are many health benefits said to be associated with the essential fatty acids, the evidence is strongest for the role of omega-3 in protecting against heart disease and possibly stroke. But the study suggests people may be getting a much lower dose than they believe.

All the products tested were within their best-before date, and price and country-of-origin were not indicators of quality. The majority of products were made in Australia or New Zealand.

A molecular pharmacologist/toxicologist with the University of Adelaide, Ian Musgrave, said of particular concern was the high oxidation levels found in some of the supplements. While getting a lower dose of omega-3 than expected probably wasn’t harmful, the by-products of oxidation could be, Musgrave said.

“That triggers a health issue,” he said. “It’s not that they’re just a little bit over the recommended levels – some are substantially over international guidelines. An excess of a breakdown product in a supplement is considered a contamination, and that could be a problem for the Therapeutic Goods Administration.”

A TGA spokeswoman told Guardian Australia the regulator was aware of the research and was reviewing this information to see whether any action was required.

“The TGA is consulting with Food Standards Australia about this,” she said. “There are currently 683 listed medicine products containing fish oil on the Australian Register of Therapeutic Goods, but also fish oil-containing products may be sold in Australia as foods, which are not regulated by the TGA.”

Dr Ken Harvey, a drug policy expert and adjunct associate professor at Monash University’s school of public health, said the study drew attention to longstanding deficiencies in Australia’s complementary medicines regulation.

“These include no pre-market evaluation of sponsors’ products against compositional quality standards or promotional claims, limited and poorly targeted post-market surveillance, and a lack of effective penalties that would deter sponsors from repeated breaches of TGA regulations,” he said.

“This is far from a recent finding; numerous reports over the last decade have recommended that the lack of effective penalties should be redressed.”

The head of the nutritional interventions laboratory at Baker IDI Heart and Diabetes Institute, Professor Peter Clifton, described the study as useful and important.

“Clearly the bulk fish oil producers have been deceiving the public and the encapsulators about the content of their oil, and the Australian Competition and Consumer Commission and the Therapeutic Goods Administration need to get involved,” he said.

People aiming to get an anti-inflammatory effect from the supplements, or who were trying to keep down their levels of triglycerides – a type of fat in the blood associated with heart disease – may have been under-dosing, he said.

“Similarly, the high oxidation products may be interfering with how well the pills work but we really don’t know the long-term implications of high oxidation products,” he said.

“As the paper points out it is possible very oxidised fish oil may promote the formation of fatty deposits in the arteries and thus the opposite of what the consumer is expecting.”

Omega-3 fatty acids are essential for body functions including blood clotting and building cell membranes. They are found in fish, including salmon and tuna, grains, including flaxseed, and some green vegetables, including kale and spinach.