Hello doctor,I happen to be losing so much...

Causes of weight loss :
1. unhealthy diet 
2. smoking
3. stress
4. medication side effects 
5. chornic diesease such as gastic disease or infections 
6. hyperthyroidism 

what to do first :
consult internal medicine doctor and do the needed tests such as :  blood test cbc and thyroid hormones tests, as well as stool culture test andl liver function test 
medication to gain 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 ?
1. vitamin C 
2. vitamin B1 / B6
3. vitamin A
general advices :
1. consult a nutritionist to guide into a better and suitable diet
2. drink more fluids 
3. make sure that your diet contain plenty of health calories
4. pay attention to dinner meal and dont miss it
5. make sure that you are not taking any medication causes weight loss as a side effect
6. exercise
7. avoid stress and anxiaty