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.

The lead made up, on average, 10 per cent of the material in the marijuana packets, boosting profits by about US$1500 ($1875) per kilogram, Franzika Busse of University Hospital Leipzig wrote in a letter to the Boston-based New England Journal of Medicine.

"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 problem was discovered last year when the first of 29 patients, aged 16 to 33, started showing up in four Leipzig hospitals with abdominal cramps, fatigue, nausea and varying degrees of anaemia.

Despite the rise of P, cannabis is showing no sign of disappearing and remains a lucrative illicit trade, police say.