Depends on what you use.
They can have side effects that can be harmless or leave a permanent scar/injury on your metabolism, entire digestive system, liver, kidneys, pancreas,...and so on. Most of them have rigorous testing so they, if used correctly, cause as little side effects as possible.
Im not a doctor or a medical student, or any type of professional in this filed.
The best advice I can give on this matter is, skip them all. You or someone you know can lose weight just by correcting his/hers diet and incorporating exercise/any type of physical activity in their life. In doing so it is best to visit your doctor to get some advice and a full checkup, and might even visit a nutritionist.
Best of luck
P.S. It is risky looking for advice/answers on the internet when it comes to your health.