Heard the latest buzz about cannabis? Word on the street is that today’s pot is exponentially more powerful, and thus more dangerous, than the marijuana available some 20, or even 10, years ago. The nation’s drug czar says so. (“We’re no longer talking about the drug of the 1960s and 1970s,” John P. Walters recently warned Reuters; “this is Pot 2.0.”) Law enforcement says so. (Speaking to the Associated Press in June, United States Drug Enforcement Agency special agent Mark R. Trouville, who heads the agency’s Miami office, said, “This ain’t your grandfather’s or your father’s marijuana. This will...