Hi Thank you for asking HCM I have gone through your query.35 kg is low weight for a female of 20 years.Ideal weight should be height in centimetres minus 100 in kilograms plus or minus five.You should rule out malabsorption syndromes and parasites by doing stool routine.Inflammatory process in body and anemia should be ruled out by doing blood routine also.Deworming should be done yearly once with drugs like albendazole.Most of weight gaining medications have lot of side effects.You can include nutritious and balanced food in your diet and adequate exercise.You can take eggs,meat,fish and milk products which helps in gaining weight.You should also get adequate sleep and non stressful life.Appetite enhancing medication can be taken in moderation if you have less appetite.Hope this may help you. Let me know if anything not clear. Thanks.