Drug dealers looking for extra profits apparently added lead flakes to packets of marijuana, inflating their value while causing dozens of cases of serious poisoning, doctors in Germany reported today.

The lead made up, on average, 10 per cent of the material in the marijuana packets, boosting profits by about $US1,500 ($A1,613) per kilogram, Franzika Busse of University Hospital Leipzig reported.

"One package contained obvious lead particles; this strongly indicated that the lead was deliberately added to the package rather than inadvertently incorporated into the marijuana plants from contaminated soil," the researchers wrote in a letter to the New England Journal of Medicine.