After alcohol and marijuana, what mood altering substance is the next most popular among U.S. high school students? You might reasonably suspect prescription pain relievers or prescription stimulants. But in a 2012 survey of drug use in the past year by 9th to 12th graders, synthetic marijuana took third place. “Synthetic marijuana” refers to a group of products that consist of dried herbs or other plant material that has been sprayed or soaked with chemicals. The structures of the chemicals mimic those of the psychoactive substances in marijuana (cannabinoids). They come from a laboratory rather than a natural source, so they are classified as designer drugs. Synthetic cannabinoids are frequently more powerful than natural ones, with a wider range of physical and mental effects—some of which can be deadly. Spice and K2 are the best-known brands. Other street and shelf names for these products include Black Mamba, Bliss, Fire n’ Ice,...