Hi Longifene Tablet is an antihistamine that is used to treat motion sickness.It is not an approved drug for weight gain and should not be used until otherwise prescribed by a doctor. The cause of inability to gain weight should be identified.Nutritional deficiency,Chronic infections,Malabsorption syndromes,Parasitic infestations, Hyperthyroidism should be ruled out. The cause should be treated along with dietary changes.Healthy diet is foremost important for weight gain. Increase your daily protein intake to see good results.Calculate your daily protein requirement based on body weight. Increase pulses, beans,eggs,curd and meat in your diet. Protein powder along with milk can be twice a day.Adequate hydration is also essential. Hope I have answered your query. Let me know if I can assist you further. Regards Dr.Saranya Ramadoss, General and Family Physician