Weight loss supplements: what the evidence actually shows

The weight-loss supplement market works because the alternative is difficult and the products are legal, cheap and confidently marketed. It is worth knowing what the trials found, because the honest summary is short.

The summary

No supplement sold in the UK has evidence of clinically meaningful weight loss. The largest pooled effects are under 1.5 kg, usually over 8–12 weeks, often from small trials, and several vanish when the weakest studies are removed.

Several carry real risk.

Going through them

Green tea extract / EGCG

A Cochrane review of 14 trials found −0.95 kg against placebo, with no significant difference in trials conducted outside Japan ^[1].

The safety signal is the more important part. A trial of 1,021 women taking an extract providing 1,315 mg of catechins daily for 12 months found significantly raised liver enzymes, some moderate or worse. There have been at least 50 case reports of liver damage since 2006, and the UK Committee on Toxicity has reviewed the issue ^[4].

Drinking green tea is not the concern. Concentrated extracts are.

Apple cider vinegar

The trial driving almost all the 2024–25 coverage was retracted by BMJ Group in September 2025 over concerns about the science and data integrity ^[2]. What remains is thin.

It also erodes dental enamel and can irritate the oesophagus.

Glucomannan

A soluble fibre taken before meals. A systematic review found that 1.24–3.99 g a day for up to 12 weeks did not significantly affect body weight against placebo ^[1].

Safety note that matters: tablet forms have caused oesophageal obstruction. Powder or capsules taken with plenty of water avoid that.

Garcinia cambogia

A meta-analysis of 12 trials found −0.88 kg — but restricted to the two rigorously designed trials, the effect was no longer statistically significant ^[1].

Ten cases of liver toxicity have been reported, including one death and two liver transplants, and three cases of mania ^[1].

Raspberry ketones

Only ever studied in combination with other ingredients. There is no isolated efficacy data at all ^[1]. That is not “weak evidence”, it is no evidence.

CLA (conjugated linoleic acid)

2.4–6 g a day produced about −0.7 kg of body weight and −1.33 kg of body fat over 6–12 months. The NIH’s own summary calls the magnitude small and the clinical relevance uncertain ^[1]. Gastrointestinal upset is common, and some isomers may lower HDL.

Berberine

Marketed as “nature’s Ozempic”, which is marketing rather than pharmacology — it is not a GLP-1 and does not act by the same mechanism. The evidence for weight loss is weak, with small, poor-quality studies ^[5].

It has genuine drug interactions through CYP3A4 and P-glycoprotein, affecting metformin, ciclosporin and statins among others, and should not be used in pregnancy or breastfeeding.

Probiotics

Covered in gut health and weight. The short version: no significant effect on body weight in the most recent meta-analysis, and the BMI and fat-mass effects do not survive sensitivity analysis.

The dangerous end

DNP (2,4-dinitrophenol). A highly toxic industrial chemical sold illegally for weight loss, usually as a yellow powder in capsules. It works by uncoupling cellular respiration, which is exactly as dangerous as it sounds — it has caused deaths in the UK. Since 1 October 2023 it is a criminal offence to sell DNP to the public without a Home Office licence ^[3].

Symptoms of poisoning include nausea, vomiting, restlessness, flushed skin, sweating, rapid breathing and irregular heartbeat. Anyone who has taken it should seek medical help immediately.

Undeclared pharmaceutical ingredients. “Herbal” slimming products bought from unregulated sellers have repeatedly been found to contain sibutramine — withdrawn from the UK market in 2010 on cardiovascular safety grounds — along with diuretics, laxatives, thyroid hormone and other actives that are not on the label.

Counterfeit weight-loss injections. A live UK problem, covered in buying weight-loss medicines safely.

What to do instead

If you want something to take, the honest answer is that the money is better spent on food that fills you up. If you want pharmacological help and you meet the criteria, that is a conversation with a prescriber about licensed medicines, which have actual effect sizes and actual safety monitoring.

When to speak to a clinician

Stop and seek medical advice if you are taking a supplement and develop:

• yellowing of the skin or eyes, dark urine, pale stools, or pain under the right ribs — possible liver injury
• palpitations, chest pain, unusual sweating or agitation
• difficulty swallowing or chest discomfort after fibre capsules

Speak to a pharmacist or GP before starting any supplement if you take prescribed medicines — particularly warfarin, statins, ciclosporin, diabetes medicines or immunosuppressants — or if you are pregnant or breastfeeding.

If you have taken DNP, seek medical help immediately and say what you have taken.

This article is for general information and isn’t a substitute for advice from your own clinician. If you feel unwell, contact your GP or NHS 111. In an emergency, call 999.