"You get what you pay for" isn't an adage we can always rely upon. A US study has found more than one-third of a selection of sports supplements bought online don't contain key ingredients the label says they should.

Pieter Cohen, a clinician-researcher at Cambridge Health Alliance and Harvard Medical School in Cambridge, Massachusetts, and colleagues ordered 57 sports supplements to analyze their contents.

Each product's label claimed the supplement contained one of five botanical compounds with purported performance-enhancing properties. The substances have been included in supplements since a stimulant called ephedra was banned in 2004.

"The FDA does not preapprove these ingredients, or any supplement ingredient, for either efficacy or safety before their introduction," Cohen and colleagues write in their paper.

"But FDA inspections have found that supplement manufacturers often fail to comply with basic manufacturing standards, such as establishing the identity, purity, or composition of the final product."

Their analysis found around 40 percent of the 57 supplements bought online (an admittedly small sample) did not contain a detectable amount of the ingredient listed. Half displayed the wrong amount, and 12 percent were found to contain illegal additives.

"Only 11 percent of products were accurately labeled and 5 different FDA-prohibited ingredients were found, including an unapproved drug available in Russia, 3 drugs formerly available in Europe, and one drug that has never been approved in any country," Cohen and colleagues report.

While quantities may vary among batches within a given brand, a recommended serving of a supplement in the study was found to contain more than three times the mass of one of the stimulants listed on the label.

Although the study findings are shocking, they aren't that surprising once you understand how supplements are regulated in the US and other countries such as Australia.

Being health products, you might think supplements fall into a subcategory of medication. But in fact, the FDA regulates them as a subcategory of food, Cohen explains.

"This has huge consequences for the whole category of dietary supplements, from vitamins, minerals, probiotics and all sorts of new ingredients," Cohen told the American Medical Association (AMA) in 2021, because "what it means is that the manufacturer can introduce anything into the [US] market that they believe is safe."

The FDA's job is then to keep tabs on new products, identify if any are causing harm, and have them removed from store shelves if so.

In 2004, for instance, the FDA banned the sale of herbal supplements containing ephedrine alkaloids "because they present[ed] an unreasonable risk of illness or injury" to consumers.

Ephedrine alkaloids, or ephedra, are stimulants extracted from Ephedra sinica and other plants, touted to increase energy and enhance athletic performance. Since their removal, ephedra poisonings in the US have dropped sharply, with no reported ephedra-related deaths since 2008.

Australia's drug regulator, the Therapeutic Goods Administration (TGA), has similarly been cracking down on supplements, which can endanger health by interfering with prescription medicines or causing serious allergic reactions.

As of late 2020, the TGA now regulates high-risk sports supplements as medicines rather than 'sports foods' after several people died.

But drug regulators are playing catch-up to a fast-moving market. Cohen says there has been an "explosion of new ingredients" in supplements in recent years, with more than 75,000 dietary supplement products sold in the US, he estimates.

"Nowadays we're seeing so many new innovations or brand-new ingredients being introduced to supplements," Cohen said in his AMA interview.

"Again, because the FDA isn't vetting these products before they show up on store shelves or on the internet, what happens is that they can pose unpredictable risks."

Further research is needed to see how many other sports supplements and health products are similarly mislabeled before we know the true extent of this problem.

However, recent studies have found melatonin gummies sold in the US and Canada could likewise give kids far higher doses than what the labels suggest.

An Australian study of 135 dietary supplements purchased between 2014 and 2017 also found only 20 percent had at least one ingredient confirmed on lab testing.

The study has been published in JAMA Network Open.