Hi! I can understand your problem of underweight. At the outset I would advise you to consult a physician for proper clinical examination & relevant lab tests to rule out the possible causes of decreased appetite like infection, nutritional deficiencies, hormonal disorders, chronic inflammatory conditions, etc. Practin (Cyproheptadine) is used as an appetite enhancer.It should be taken under the prescription of a physician to avoid misuse and addiction No doubt it helps in gaining the body weight by increasing the appetite, but in the long run it may cause side effects like drowsiness, dry mouth, constipation, inability to pass urine, blurring of vision, increased heart beats, etc. What you need most essentially is life style modifications. Avoid stress, do regular exercise this may increase your appetite also. You should take healthy balanced diet while you are on this drug. There will be tendency to take unhealthy foods like excess fat or sugar. You should have a control over it. Plenty of fluids also should be taken. You should also avoid taking carbonated and soft drinks. If this answers your query, kindly rate this answer.