Is there any meds that can help you gain weight?

Not if one's healthy: A doctor can check for digestive, hormonal, metabolic, infectious, or cancerous conditions in a person "looking too thin". Hopefully, no problems are found. Choosing proper designs of clothing and shoes can help one appear bulkier. To increase muscle mass, one has to work out with weights. To add a little fat (if one is "skin & bones"), one must eat a larger amount of his healthy, balanced diet.

Specific medication: are not needed to gain weight. However, if you have been losing weight & are ill, that warrants evaluation. Eat healthy foods high in calories. Eat 6 meals /day. Work on strength training to build large muscle groups. Work out w heavier weights w fewer repetitions. Sleep at least 8- 9 hrs/night. Hydrate well w water. Foods to help w weight gain: almonds, cashews, pecans, sunflower seeds, flax seed