Hi,
1. What is the body frame of your parents, siblings, if any.
2. If they are also on leaner or smaller frame, then you may not be able to increase the weight. As no medicine can increase the inherited number of muscle or fat cells. ( Like we can not increase fixed capacity of hard disk of mobile or computer)
3. At the most, we can increase little bit mass.
4. Accept the reality. Be happy
5. You are blessed with good metabolism, you will have very less chances of having
diabetes or blood pressure ( Based on the observation of many such personality)
Health Tips
Do regular weight training followed by having fistful almonds or peanuts , if not allergic. Or have protein shake.