Fact check: do diet pills work?

With around two-thirds of adults in Britain classified as overweight, there is a growing appetite for slimming pills that promise rapid weight loss.

But do the claims behind them stack up? The Week investigates.

What drugs are available and how do they work?

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Pharmacies and health shops stock dozens of weight-loss aids, most of which are designed to be taken by people who also make sensible lifestyle changes.

Some slimming aids claim to work by suppressing appetite or speeding up the metabolism, while others disrupt fat absorption.

There is currently only one weight-loss drug available on prescription in the UK. This drug, orlistat, is typically given to people with a body mass index (BMI) of 28 or more who have already made a significant effort to lose weight and are following a balanced diet and exercise programme.

The medication works by interfering with the way that fat is digested, and prevents around a third of the fat from food being absorbed by the body, according to the NHS.

A reduced-strength version of orlistat, marketed as Alli, is available without a prescription.

A much wider range of prescription diet pills are available in the US. As well as orlistat, the Food and Drug Administration (FDA) has approved bupropion-naltrexone (Contrave), liraglutide (Saxenda), phentermine-topiramate (Qsymia) and lorcaserin (Belviq) for long-term use.

Lorcaserin, an appetite suppressant, has been hailed by some as the “holy grail” in the fight against obesity, but has yet to be approved by EU regulators.

What does the research say?

A review carried out by NHS clinicians and experts at the Royal Pharmaceutical Society (RPS) found that most over-the-counter slimming aids had little or no published medical evidence to support their weight-loss claims.

The 2010 study, conducted on behalf of the BBC, looked at five popular products available on the UK high street: Adios, Biosynergy Hoodia Gordonii, LIPObind, Slim Nite and Alli.

It concluded that Alli was the only slimming aid backed by robust research. “For the four others investigated, there are no published randomised clinical trials to support their efficacy in weight loss,” said Dr Colin Cable of the RPS.

Dietician Ruth Kander, who also took part in the review, said: “People want a quick fix in their life, they want to lose weight really quickly, and they don’t want to give up their favourite foods.

“But that’s not real life and that’s not what clinical practice is about. And some of these products haven’t got the research which is needed to see that they’re effective.”

What about prescription medications?

The NHS says that orlistat is the only anti-obesity medication that is proven to be safe and effective, but that it must be combined with a calorie-restricted, low-fat diet and regular exercise.

A large-scale Swedish study published in 2004 found that orlistat combined with a healthy diet and exercise helped patients lose almost twice as much as lifestyle changes alone.

Researchers also found that people who took the drug had a lower incidence of type 2 diabetes at the end of the four-year study.

A separate review study published in 2014 found that adults taking orlistat had an average 12-month weight loss that was 7.5lbs (3.4 kg) greater than those on the placebo.

However, this amounted to just 3.1% of the subjects’ initial weight, “which is not particularly impressive”, argues Joe Leech, an Australian dietitian, in an article for Healthline. It also appears that weight is slowly regained after the initial year of treatment, he says.

In addition, experts warn that orlistat can cause some unpleasant gastrointestinal problems, including faecal incontinence, flatulence and oily rectal discharge.

“Although this is an effective weight-loss drug, its side effects include oily diarrhoea, which can cause great discomfort,” Dr David Haslan, chair of the National Obesity Forum, told Men’s Health.

Lorcaserin, which is licensed for use in the US but not the EU, has also delivered some promising results, albeit from research funded by pharmaceutical company Eisai, which owns the rights to the drug. The September 2018 study found that adults using lorcaserin lost an average of 8.8lb (4kg) over 40 months.

Researchers also found that people taking the drug did not face an increased risk of heart attack or stroke, as is the case with some weight-loss medications.

But the NHS notes that the drug was part of a combined treatment plan that also included dietary changes and exercise, so people “should not expect to rely on the drug alone” for weight loss.

“Also, the results in terms of weight loss were modest,” adds the health service’s website. “Most people taking the drug were still in the overweight or obese category by the end of the study.”

What’s the consensus?

Most over-the-counter slimming aids are not supported by clinical research, experts conclude.

However, the evidence suggest prescription anti-obesity drugs such as orlistat and lorcaserin are safe and can produce modest weight loss compared with a placebo when combined with lifestyle changes.