You have to keep in mind that everything has side effects, some intentional and some from misuse of a product. There can be sauna belt side effects if used improperly. It is not supposed to be used for longer than 50 minutes per day. Using it longer can cause some burning of the skin or just skin irritation. You should feel that you are overdoing quite easily and remedy the situation without any problem by taking it off. If worn too long blisters could develop as they would from any hot object coming into contact with your skin for too long a time span.
The belt can have positive effects for your body if worn the appropriate amount of time. All those who have had physical therapy know that before the application of ice, the program usually begins with a heat wrap. Some have claimed, without positive proof, that the sauna belt side effects include damage to a woman's kidneys and reproductive organs. Also unproven has been the claim that men have had fertility issues because of the heating of the sperm, due to a high temperature in the groin area from the belt.
Sauna
If you would like to use the sauna belt follow the directions for how long to use it and how it should be applied. Do not exceed any of the recommendations. They are made for your comfort and safety. If your skin is sensitive you might have side effects that others will not experience. Use it like everything else, with caution.
Just use a sauna belt correctly and you will not have to worry about sauna belt side effects.