Heroin is an extremely dangerous drug that kills many people every day. I would not recommend using it, even if it does increase appetite.
I suffered from addiction years ago, and although I never became addicted to heroin, I tried it several times, and it never increased my appetite, in fact it had the opposite effect on me.
Unless you are receiving advice from a doctor for ways to increase your appetite, you might get advice on the internet that is detrimental to your health, and well being.
For me Marijuana increases my appetite, and I use it for medicinal purposes. The increased appetite is a side effect of the medicine, but it's a definite side effect that many people will concur to. I'm not saying you should smoke pot though. I am not recommending you take any drugs without doctors orders.
My best advice is don't use heroin. It's a dangerous street drug. Consult with a doctor for the best advice regarding your appetite.
I hope my answer helps.