In a crowded Washington DC bar one recent Friday night, a patron carrying a nefarious-looking black plastic bag pushes his way past bearded hipsters and preppy Capitol Hill staffers, stopping at a table packed with weekend revelers. “Check this out,” he says, holding up the bag. He pulls out a tall colorful can of Four Loko, a fizzy-lifting drink loaded with caffeine and booze that has made headlines recently when it was banned by the Food and Drug Administration. Nearly everyone at the table lunges to touch the taboo cocktail, pulling it in close to finally catch a glimpse of...