Why are heroin addicts so skinny?

Part of the effect is from the drug itself and the other part is from the long term effects of the drug. The high itself makes you really euphoric, relaxed, and tired, none of which are particularly conducive to an appetite. The other reason is that often once you get seriously addicted to it, you don't really focus on anything other than where your next high is coming from. It's kinda like that for most highly addictive drugs that cause intoxicating effects.