The gray matter of the nucleus accumbens, the walnut-shaped pleasure center of the brain, was glowing like a flame, showing a notable increase in density. “It could mean that there’s some sort of drug learning taking place,” speculated Jodi Gilman, at her computer screen at the Massachusetts General Hospital-Harvard Center for Addiction Medicine. Was the brain adapting to marijuana exposure, rewiring the reward system to demand the drug?

Dr. Gilman was reviewing a composite scan of the brains of 20 pot smokers, ages 18 to 25. What she and fellow researchers at Harvard and Northwestern University found within those scans surprised them. Even in the seven participants who smoked only once or twice a week, there was evidence of structural differences in two significant regions of the brain. The more the subjects smoked, the greater the differences.

Moderate marijuana use by healthy adults seems to pose little risk, and there are potential medical benefits, including easing nausea and pain. But it has long been known that, with the brain developing into the mid-20s, young people who smoke early and often are more likely to have learning and mental health problems. Now researchers suggest existing studies are no longer sufficient. Much of what’s known is based on studies conducted years ago with much less powerful pot.

Marijuana samples seized by the federal Drug Enforcement Agency show the concentration of THC, the drug’s psychoactive compound, rising from a mean of 3.75 percent in 1995 to 13 percent in 2013. Potency seesaws depending on the strain and form. Fresh Baked, which sells recreational marijuana in Boulder, Colo., offers “Green Crack,” with a THC content of about 21 percent, and “Phnom Penh,” with about 8 percent. The level in a concentrate called “Bubble Hash” is about 70 percent; cartridges for vaporizers, much like e-cigarettes, range from 15 to 30 percent THC.