Here are some possible side effects that even the best fat burners could have and some precautions you can take to avoid them:

Allergies: The most common symptoms are skin allergies, itching, gastric upset, etc.
Insomnia or poor sleep: The stimulants (caffeine) found in fat burners alter sleep patterns, which can also increase your heart rate and lead to sleep deprivation.
Increased blood pressure: The result of an accelerated metabolism can cause the heart to pump faster than it should, which can increase your blood pressure.
Changes in behavior: Mood swings, increased anxiety, increased blood pressure, emotional disturbances, irritability, aggression, and nervousness are also common. Stomach problems: Fat burners can cause stomach problems such as gas or bloating, diarrhea, constipation, and other bowel problems, and sometimes black stools with rectal bleeding.
Nutrient deficiencies: Excessive caffeine consumption can lead to deficiencies in certain vitamins such as vitamin B12 and iron.