Cyproheptadine
"I've always been quite underweight and absolutely hated it, tried this years ago and quit a few pills in due to the tiredness caused. Gave it another shot 6 weeks ago and have gained a stone already (usually impossible!). 

 I've been taking one a day which is a low dose and my appetite has easily doubled. I used to feel full half way through a meal and feel anxious going out for food in case I couldn't finish it. Definitely not a problem anymore. I've given it a 10 as it has made such a huge impact on my self esteem already, and unexpectedly resolved what I believe to be urticaria over night. The only downside is the tiredness; makes getting to sleep nice and easy but waking up is so difficult! Long way to go still, but very happy so far :)"