Dr. Marylin Huestis of the National Institutes of Health said the study involved 30 male daily marijuana smokers who temporarily lived in a secure research unit for up to 33 days. Their blood was collected daily. She said 27-of-30 participants were delta-9-tetrahydrocannabinol, or THC positive on admission. THC is the main psychoactive chemical in marijuana.
The THC decreased gradually with only 1-of-11 participants negative at 26 days; while 2 of 5 remained THC-positive for 30 days, Huestis said.
"These data have never been obtained previously due to the cost and difficulty of studying chronic daily cannabis smoking over an extended period," Huestis said in a statement. "These data add critical information to the debate about the toxicity of chronic daily cannabis smoking."
The findings were published online in Clinical Chemistry.