Since the late 1970s, researchers have recorded significant increases in THC concentrations in confiscated marijuana, says Susan Weiss, PhD, chief of the Science Policy Branch for the National Institute on Drug Abuse.

In the mid-1980s, average THC content in marijuana seized by the federal Drug Enforcement Administration was around 3.6 percent, while last year, THC content was closer to 10 percent, Weiss says. The highest THC content recorded in the program last year was 32 percent, she says.

Researchers don’t know whether a greater proportion of first-time users encountering marijuana in its stronger forms will develop dependency, says Weiss, but the question needs to be investigated.

A study in the Netherlands on the smoking habits of 388 marijuana users identified three broad types of cannabis users: the “strongest high” type, younger smokers who inhaled marijuana smoke more deeply; the “consistent high” type, who compensated for stronger marijuana by inhaling less deeply and smoking less; and the “steady quantity” type who smoked to a intermediate dose and stopped. The study, published in May 2007 in the International Journal of Drug Policy (Vol. 18, No. 3), found that younger smokers scored highest for psychological dependency.

“If they’re starting out possibly exposing themselves to higher doses, then there may be more adverse consequences and, possibly, more dependency,” says Weiss. “But we don’t really know that there’s a straightforward relationship there.”

—C. Munsey