There are various reasons of not gaining weight or reason of being underweight
1. Endocrine disturbances
2. Genetic
3. Vitamins and minerals deficiency
4. Psychological problem
5.IBS
6. Eating disorder
7.
Liver dysfunction
And many more
See, we cannot prescribe like this. Before prescribing anything we need some information like your vitals, health parameters and manymore
Next Steps
For more information and to contact me directly visit https://www.facebook.com/pg/doctorplanmydiet/about/
Thanks with regards
Dt.Ankita