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I'm not a heroin user, but what I've read, powder is better than tar. Potenty, texture wise, and its less complicated to shoot ( I think). Also, pure heroin is white powder, although I'm sure your not getting pure. But good dope can be white or brown, or off white I think. And thats awesome if he's letting you try it out first, theres really nothing to worry about then. If it sucks, don't buy it.
"When you can stop, you don't want to. When you want to stop, you can't. "