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The investigation into medical devices has uncovered all kinds of anomalies – and one of the more startling involves diet pills.

To the millions of people who buy them to lose weight, the pills are exactly that – capsules to swallow.

But EU regulators have taken a different view. To them, the pills are not drugs that need to be tested in the usual manner, they are classified as “medical devices” – and as such, they can get to customers more easily.

This has led some experts to warn that the pills – some of which rapidly expand in the stomach “like a balloon” to up to 200 times their initial size – are getting safety approval through the backdoor.

Under EU rules, a medical device can be any product used for the diagnosis, prevention or treatment of a disease, that does not use “pharmacological, immunological or metabolic means” alone to achieve its aim.

Food supplements, on the other hand, are defined as “concentrated sources of nutrients (or other substances) with a nutritional or physiological effect”.

Among the diet aids that are marketed with a CE (Conformité Européenne) mark, which indicates they meet EU safety standards, are pills containing dietary fibre that promise to swell up in the stomach. These can be found on the shelves of high-street chemists and supermarkets.

Critics of the system say manufacturers are able to obtain safety certificates without the kind of testing that would be required if the products were classed as drugs, and are allowed to make claims they would not be able to make if the products were classed as food supplements.

Dr Matt Capehorn, the clinical manager at the Rotherham Institute for Obesity, said manufacturers were using the rules to get safety approval through the back door. He said that while product approval through the pharmaceutical route involved testing on “hundreds if not thousands of people, it can be just 10 for a medical device”.

He added: “If any of these companies are confident about their products, why don’t they put them through a big randomised control trial to prove they work.”

As long as customers follow the instructions, there is no suggestion there are safety issues with these types of pills.

One manufacturer that has had products CE-marked as medical devices is XLS Medical. The products include Hunger Buddy, which comes in capsule form and “swells up to 200x its original size like a balloon and forms a thick, indigestible gel which fills up to 30-43% of the stomach”. A pack of 40 costs £15.

Another XLS Medical product is Fat Binder, priced at £60 for a month’s supply, which contains the natural fibre complex Litramine. This “binds fat in your stomach to form a complex too large to be absorbed by the body … XLS Medical Fat Binder also contains vitamins A, D and E to help you reinforce your body,” a product description says.

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XLS Medical declined to comment on its products and referred questions to its trade body, the Proprietary Association of Great Britain (PAGB).

The PAGB said: “For self-care medical devices, the key distinction is that medical devices have a physical action on the body and medicines, in comparison, have a pharmacological action.”

Boots sells its own Appetite Control capsules, which also carry a CE mark. Appetite Control, sold in packs of 60 for £16.99, “creates a pleasant feeling of fullness, helping you feel satisfied more quickly during a meal, and helps to maintain this feeling after a meal”, according to a product description.

Each capsule contains “500mg of a specialised high swelling capacity fibre complex of plant origin, potato starch, hypromellose (E464), sunflower oil, titanium dioxide”.

A spokesperson for Boots UK said: “The health and wellbeing of our patients and customers is extremely important and we take our responsibility seriously to ensure products are labelled correctly.

“Our Boots Appetite Control product has a physical mode of action in the stomach and is therefore regulated as a medical device. We fully comply with all legal requirements and current guidelines for the correct labelling of medical devices as set by the MHRA [Medicines and Healthcare Products Regulatory Agency].”

The EU definition of a medical device will not change under new rules that come into force in 2020.

The MHRA, which regulates the UK’s medicines and medical devices industries, said it had previously raised concerns about whether the rules for devices should be applied to this type of product, and had pushed the EU to make changes.

A spokeswoman said that during negotiations on the new medical devices regulations, the MHRA secured amendments to the legislation. “We believe that these changes will place an appropriate level of additional scrutiny on these products to ensure their safety and clinical effectiveness,” she said.