All fat burners can be divided into 2 groups:
The sports nutrition category includes thermogenics (increase body temperature) and lipotropics (L-carnitine, CLA, omega-3, choline, melatonin, chromium picolinate). The latter are considered safer, but less effective. Thermogenics are more active in burning fats, but contain more aggressive substances that have more side effects than lipotropics.
Both thermogenics and lipotropics show maximum efficiency with regular exercise. If you do not exercise and continue to eat improperly, the result may be, but very small.
Pharmaceutical drugs include:
Many people still use diuretics for weight loss. But they do not burn fat, but only remove excess fluid.
Serious pharmaceutical drugs are used to treat obesity (begins with a BMI over 30). Moreover, they are prescribed only when diet and exercise do not work.
Many of these drugs have serious side effects and have an exciting effect on the body. Therefore, they are used in short courses, which may be ineffective in the fight against excess weight.
Because, together with the intake of these drugs, you need to go in for sports for the effect to be, and overweight people are mostly lazy. L-carnitine works well, in the SportExperta series it is produced in liquid form, therefore it is better absorbed. But you need to drink it before training, it increases stamina and promotes weight loss.
With all due respect, but to say that fat burners work, will only tell a fitness trainer with his shop of this potion.
Chemistry that will drain the water - no problem. But to burn fat? How's that? - well, at least in a nutshell. The principle of action.
As far as I know, there are no substances that can burn something without training and diet, because there are gyms and fitness centers as a phenomenon. (Otherwise, there would be no point in the entire industry)
Therefore, answering the question - a fat burner cannot be harmful or useful, simply because of its absence. There is a name. There is packing. There is advertising. There is a high price. No fat burning substance.
There are different fat burners. There are safe, but ineffective, and there are the most effective, but potentially hazardous to health.
The topic is quite extensive and it is impossible to cover it completely in the format of the answer to the question, so here is a link to a more or less complete overview of the types of fat burners, the principles of their work and the danger and safety for the body.