Theoretically you COULD overdose on cannabis... but I doubt anyone could actually do it. If we assume that the LD50 (the dosage that will have a 50% chance of killing you) for injected thc is the same for humans as for monkeys (although it is probably higher), it would be 128mg/kg, or about 58mg/lb. So for a 150lb person you would have to inject 8700mg, or 8.7 grams, of PURE thc to have a 50% chance of dying. And mind you you would have to inject it all at once, because thc is absorbed by serum lipids extremely quickly and that prevents toxicity. Also that is only for injected thc, taking it orally or smoking it would require over 6 times as much. So really, it is pretty much impossible to overdose on weed unless you are really REALLY trying to, by injecting over a quarter ounce of pure thc into your bloodstream through pretty much every mainline vein you have.