Red and purple blossoms with fat, opium-filled bulbs blanket the remote creek sides and gorges of the Filo Mayor mountains in the southern state of Guerrero. The multibillion-dollar Mexican opium trade starts here, with poppy farmers so poor they live in wood-plank, tin-roofed shacks with no indoor plumbing. Once smaller-scale producers of low-grade black tar, Mexican drug traffickers are now refining opium paste into high-grade white heroin and flooding the world’s largest market for illegal drugs, using the distribution routes they built for marijuana and cocaine. The price for the relatively low-quality marijuana the farmers used to grow at lower elevations has fallen, possibly because of the legalization and medical use of higher-quality U.S. marijuana.