Hi

medication to gain weight : increase weight /
- there is some simple medication that can increase your appetite thus increasing your weight such as :
1. Dronabinol 
2. Megestrol 
3. Oxandrolone
- first consult your doctor before taking any of those medications 
- Vitamins that help with increase weight:  vitamin C   vitamin B1 / B6 vitamin A
Causes of weight loss :
1. unhealthy diet 
2. smoking
3. stress
4. medication side effects 
5. chornic diesease 
general advices :
- consult a nutritionist to guide into a better and suitable diet and drink more fluids  , plus  make sure that your diet contain plenty of health calories and  pay attention to dinner meal and dont miss it , and make sure that you are not taking any medication causes weight loss as a side effect, on the other hand , you have to  exercise regularly and avoid stress and anxiaty