No matter how much food I eat I can't seem to gain weight am 18 and I weight 120lbs also am 5 11" I did my bmi the other day and found out I was under weight.. I just want to gain weight so bad but I don't know what else to..

See your doctor.

You may have a thyroid condition that's preventing you from weight gain. That's just a guess. We can't give a diagnosis over the internet.

Your best bet is your doctor.

Good luck.

I'm 30/m and I have the same problem.. during my early 20's I wanted to serve in the military, and got delayed 3 months while my recruiter had me on a strict 10,000/day calorie diet.. I didn't put on a pound and was given a waiver. I later saw a nutrtionalist who again focused mainly on increasing my daily intake with no results. If you figure this out tell me because I don't get it. And I've already wasted $1000's on weight gainers so you may want to skip that step..