These results differ from the comparison of marijuana use and marijuana use disorders among US adults between 1991–1992 and 2001–2002.28 In that comparison, the prevalence of marijuana use was stable, while the prevalence of marijuana use disorders increased. Thus, the increase in risk for marijuana use disorders occurred only among users. This increase among users was attributed to the increasing potency of Δ-9-THC in marijuana28 between the 2 surveys, assuming that greater potency increases reinforcing effects. However, while Δ-9-THC potency in marijuana continued to increase from 2001–2002 to 2012–2013,32,33 the risk for marijuana use disorder among users decreased. Perhaps for marijuana, once a certain strength is reached, further potency no longer increases the reinforcing effects, an explanation consistent with findings that marijuana users titrate smoking behaviors, eg, inhalation, to compensate for potency.52,53 Further research is warranted to clarify this issue. Explanations for the decreased prevalence among users may also include greater medical or experimental-only users in 2012–2013. These and other explanations merit investigation.
