They had stomach cramps, nausea, anemia and fatigue, and some even had a telltale bluish line along their gums  classic signs of lead poisoning. But the cases, last year in Leipzig, Germany, puzzled doctors. Lead poisoning is rare in Germany, and yet here were 29 cases in just a few months. The doctors noticed a pattern: the patients were young, from 16 to 33; they were students or unemployed; and they had body piercings and a history of smoking.

In a letter published Thursday in The New England Journal of Medicine, the doctors wrote, “On questioning, all the patients eventually conceded that they were regular users of marijuana.”

Three provided samples for testing. Sure enough, their marijuana was full of lead. One bag bought from a dealer even contained lead particles big enough to see, which meant the lead must have been added deliberately, rather than being absorbed into the plant from contaminated soil.

The core temperature of a joint can reach nearly 2,200 degrees Fahrenheit, the doctors noted  more than hot enough to melt lead, which can be absorbed through the respiratory tract.