New research shows modern, more potent strains of marijuana may be especially bad for young recreational pot users: 16-25 year olds who smoke once or twice a week. Early studies show it can change the developing brain. Scientists are focused on conducting new research since pot has changed so much since the 1990s, when pot only contained 3 or 4 percent of THC, the chemical that gets you high. But now, some varieties can have up to 30 percent of THC. And new research is showing concentrated consumption can be hard on the developing brain.