Hello, Welcome to HCM I understand your concern. I looked up Endura Mass™ . It is a protein rich and low fat food supplement which has healthy constituents and vitamins on the label. I do not anticipate any significant side effect if the constituents are only as on the label. The best way to gain weight is to eat a wholesome food rich in calories. You could use healthy fats as these are higher in calories when compared to carbohydrates and proteins. You should also check to exclude any problems like malabsorption , hyperthyroidism, diabetes or any wasting diseases. You may use appetite stimulants if the appetite is poor. Avoid steroids as they are harmful in the long run. If you have further queries, you are welcome to ask. Best wishes, Dr. Noble Zachariah