The evidence-led answer
NST verdict
Mostly no. "Fat burner" is a marketing label, not a type of drug. These tubs bundle a dozen ingredients, each with a story. Of all of them, only caffeine has reasonable evidence for a small, dose dependent effect on body fat, and you can get caffeine from coffee for pennies. Green tea extract shows a weight difference of essentially zero in good reviews, not clinically meaningful. The rest, L carnitine, CLA, forskolin, chromium, kelp, and the newer names, have weak or no supporting evidence. Meanwhile the downsides are real: high dose stimulant blends raise your heart rate and blood pressure and wreck your sleep, and concentrated green tea extract has been tied to rare cases of liver injury. Fat loss comes from a sustained calorie deficit. A fat burner, at best, nudges it slightly; at worst it costs you money and, occasionally, your health.
What is a "fat burner"?
A category invented by supplement marketing, not by science. "Fat burner" covers anything claimed to speed up fat metabolism, raise energy expenditure, block fat absorption, or otherwise promote fat loss. A typical product stacks many ingredients, caffeine, green tea, carnitine, conjugated linoleic acid (CLA), forskolin, chromium, kelp, fucoxanthin and more, each with its own proposed mechanism, and claims that together they add up. A review of the field put it bluntly: the ingredient list is industry driven and grows faster than the science behind it.
Does anything in them work?
Caffeine, a little. A pooled analysis of 13 randomised trials found caffeine intake was modestly and dose dependently associated with lower body weight, BMI and fat mass, roughly a fifth to a quarter more reduction for each doubling of the caffeine dose. That is a real effect, but a small one, the studies vary enormously, and caffeine is in coffee.
Green tea extract, essentially not. A Cochrane review of trials conducted outside Japan found a mean weight difference of about -0.04 kg, statistically not significant, and, in the reviewers' words, "not likely to be clinically important". It also had no effect on keeping weight off. Everything else, L carnitine, CLA, forskolin, chromium, kelp, fucoxanthin, has little or no good evidence as a fat loss aid, and the newer names on labels such as raspberry ketones and garcinia are not better supported.
Are they safe?
Often less than they look. The stimulant load in many fat burners, large doses of caffeine plus other stimulants, can raise heart rate and blood pressure and badly disrupt sleep, which itself works against fat loss. Concentrated green tea extract, very different from drinking tea, has been linked to rare cases of liver injury; among the green tea trials that recorded adverse events, two involved hospitalisations. "Natural" on the label does not mean risk free, and these products are not regulated like medicines.
What this means in practice
There is no supplement that replaces a calorie deficit; fat loss comes from eating a bit less than you burn, sustained over time. If you want caffeine's small edge, a coffee before training is cheaper, more predictable and safer than a proprietary blend where you cannot see the doses. Skip the other ingredients; on the evidence they are paying for a story. Check with a doctor first before taking any stimulant supplement if you have a heart condition, high blood pressure, are pregnant or breastfeeding, or take other medications. And if weight or food has become a source of distress, that is worth talking through with a registered dietitian or your doctor, more than any product will help.
Practical meaning
No supplement replaces a sustained calorie deficit, which is where fat loss comes from. If you want caffeine's small edge, a coffee before training is cheaper, more predictable and safer than a proprietary blend; skip the other ingredients. Check with a doctor before any stimulant supplement if you have a heart condition, high blood pressure, are pregnant or breastfeeding, or take other medications. If weight or food has become a source of distress, speak to a registered dietitian or your doctor.
Limitations and uncertainty
The main review is from 2011 and the ingredient list has grown since, though its core conclusion, that only caffeine, and weakly green tea, have support, is expected to hold. The caffeine pooled analysis has very high between study variability and reports a dose response coefficient, not absolute kilograms. The green tea weight estimate is from trials outside Japan; Japan trials were more favourable but could not be pooled. The green tea extract liver injury signal is recognised by regulators; the article does not give a full safety assessment.