I definitely think this is about either coke or meth. "only a line" and "now it's a circle" suggest that because when you do coke or meth, you start with a line, feel good, and then you chase that first good feeling all night until you run out, which leads to addiction. The chase never ends, just like a circle. "the poison's in the dosage." too much drugs is poison. "You said you can't get to heaven unless you get high, but there's no word of hell on the way back down" getting high feels like heaven, till you're coming down off your buzz when it gets worse and worse. "It all looks so much worse on the way back down" well, it does. This could be talking about coming off an addiction, where you truly see the evil of the drug you were addicted to, or just coming down when you feel like s*t. So yeah, def about some coke or meth. Bad oli.