See, I would think they would be able to smoke marijuana and get high effectively.  Because of the amount of interaction between the blood and the lungs.  THC, the chemical of importance in marijuana, is very fast acting.  Though honestly I would consider the act a waste for them since their bodies would quickly dispose of the chemical.  So a brief, potent high might be reached followed by a sudden case of the munchies as their body resisted and then followed with the crap feeling afterward.  They would probably have to smoke a noxious amount of the drug for any prolonged affect as they sought to saturate their blood with THC.<br/><br/>