Do weight loss supplements work? What the evidence says, the risks of fat burners, and what actually helps

What actually helps you lose weight

What works is a modest calorie deficit, regular activity, enough sleep and time. NIH ODS, the US government office for dietary supplements, describes lifestyle change as the basis of long-term weight loss. No supplement replaces it.

Weight is about health and how you feel, not how you look. Promises of fast weight loss are a warning sign, not a reason to buy. If you live with obesity or weight-related health problems, start with your GP: proven treatments exist.

What to expect from a supplement:

• a fibre such as glucomannan may help a little with fullness, and only alongside a calorie deficit;
• no tub can burn fat, speed up your metabolism in any noticeable way, or replace what you eat;
• some fat burners strain the liver and heart, and some have been found to contain hidden medicines.

Do weight loss supplements work? The evidence

Key findings from the NIH ODS reviews of popular ingredients:

For scale: your weight can shift by a kilogram or more within a single day. Here are the same figures side by side.

Glucomannan and fibre: the only approved claim

Glucomannan is a soluble fibre from the konjac root. It swells in the stomach and helps you feel full. In 2010, EFSA accepted that glucomannan contributes to weight loss, but only under strict conditions:

1. 3 g a day, as three 1 g doses.
2. Each dose taken with 1–2 glasses of water.
3. Taken before meals.
4. Only alongside a calorie-reduced diet.

Even then the effect is modest, and a 2015 review of 6 trials found no difference from placebo. On safety, 7 cases of tablets blocking the gullet were reported in Australia in the 1980s. Capsules and powder haven’t been linked to this, but always take them with plenty of water. Glucomannan isn’t suitable if you have trouble swallowing.

Other fibres, such as guar gum and beta-glucans, didn’t change weight in trials. Fibre still helps your gut and fullness, as covered in our guide to psyllium and fibre supplements.

Green tea extract and your liver

Green tea extract is a common fat-burner ingredient. Its main active compound is a catechin called EGCG. What’s known:

• EFSA (2018): EGCG from supplements at 800 mg a day or more may cause early signs of liver damage. EFSA couldn’t identify a safe dose for supplements.
• A Minnesota trial: women taking 843 mg EGCG a day for a year had raised liver enzymes, some of them markedly.
• Since 2006, at least 50 cases of liver injury linked to green tea extracts have been reported.
• Brewed tea is considered generally safe by EFSA, with no sign of liver harm even at high intakes.

There’s no authorised weight claim for green tea in the UK or EU. If you like green tea, drink it as tea.

Caffeine and fat burners

Many fat burners contain caffeine. It may have a small effect on weight in short trials, but your body quickly gets used to it.

EFSA’s safe levels for adults (2015), from all sources combined:

• up to 200 mg in a single dose;
• up to 400 mg a day;
• in pregnancy, up to 200 mg a day.

Add the caffeine from a fat burner to your coffee, tea and energy drinks, and it’s easy to pass 400 mg. Check the caffeine per serving on the label and count your drinks too: the morning coffee, the tea at work and the fat burner all go into one total.

Chromium, berberine, CLA and garcinia

• Chromium picolinate. A Cochrane review found 1.1 kg over 12–16 weeks, from low-quality evidence. Details in our chromium picolinate guide.
• Berberine. NCCIH calls the weight evidence not conclusive, and berberine interacts with many medicines. See berberine benefits and risks.
• CLA (conjugated linoleic acid). 0.7 kg over 6–12 months. One form may worsen insulin resistance in men with obesity.
• Garcinia cambogia. Ten cases of liver toxicity have been reported, including one death and two liver transplants. Most products contained other ingredients too, so garcinia’s role is hard to prove, but the risk is there.

Apple cider vinegar gummies

The most quoted study on apple cider vinegar was a Lebanese trial published in BMJ Nutrition, Prevention & Health in 2024. BMJ Group retracted it in 2025 after finding implausible statistical values, doubts about the raw data and no prospective trial registration.

We found no other reliable evidence that apple cider vinegar helps with weight. If you buy gummies anyway, check how much sugar each serving contains.

Hidden drugs in slimming pills

This is the most serious risk. Slimming products sold as natural sometimes contain prescription or banned medicines:

• The US FDA found sibutramine in “bee pollen” slimming capsules, and warns that such products are often labelled “all natural”.
• Sibutramine was taken off the market in 2010 for safety reasons. The FDA notes it can substantially raise blood pressure or pulse in some people, a serious risk with heart disease or a history of stroke.
• The MHRA reported that in 2015 it seized over 240,000 doses of unlicensed slimming pills and closed more than 2,000 unauthorised online sellers. It warns that such pills can contain dangerous ingredients.

Buy only from reputable sellers, skip anything promising rapid results, and never order slimming pills through social media.

Weight loss medicines: only through a doctor

Medicines with proven effects on weight exist, such as semaglutide (Wegovy) and tirzepatide (Mounjaro), which act on GLP-1. They aren’t supplements:

• only a registered healthcare professional can prescribe them;
• they’re not for use in pregnancy, when trying to conceive or while breastfeeding;
• nausea, vomiting and diarrhoea are among the most common side effects, and pancreatitis, though infrequent, has been reported;
• they’re used alongside diet and activity, not instead of them.

On the NHS, your GP can tell you whether you meet the criteria for these medicines. If you get one privately, the MHRA says to use a legitimate, registered pharmacy to avoid falsified pens.

When food and weight feel hard

If thoughts about food and weight take up a lot of your day, you often feel guilty after eating, eat in secret, or feel out of control around food, you’re not alone. Help works. Talk to your GP, or contact Beat, the UK eating disorder charity, which runs a helpline. Slimming supplements can make things worse in this situation.

Who shouldn’t take fat burners

• Pregnant and breastfeeding women.
• Children and teenagers, unless a doctor advises a specific treatment.
• People with heart disease, an irregular heartbeat or high blood pressure, because of caffeine and other stimulants.
• People with liver disease, because of green tea extract, garcinia and blends.
• People on regular medicines, especially for diabetes, blood pressure or depression.

Vitamin and mineral combinations can be checked in our compatibility chart.

How to choose a weight loss supplement on iHerb, if you decide to

1. Start with food, activity and sleep. If you live with obesity or related conditions, see your GP first.
2. If you want a supplement, choose glucomannan capsules or powder and follow EFSA’s conditions.
3. Avoid green tea extract capsules and multi-ingredient fat burners.
4. Check the caffeine per serving and add up your drinks.
5. Don’t buy anything that promises fast results or hides its full ingredient list.

If a tub is already in your basket, check it against these six points:

More supplements in this section are in energy and heart. Not sure which vitamins you actually need? Try our vitamin check.