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May 2, 2012

Long-term Marijuana Use and Pulmonary Function—Reply

Author Affiliations

Letters Section Editor: Jody W. Zylke, MD, Senior Editor.

Author Affiliations: Department of Epidemiology and Biostatistics, University of California, San Francisco (Dr Pletcher) (mpletcher@epi.ucsf.edu); and Birmingham VA Medical Center, Birmingham, Alabama (Dr Kertesz).

JAMA. 2012;307(17):1796-1797. doi:10.1001/jama.2012.3607

In Reply: As noted by Drs Patel and Khazeni, accurately measuring long-term exposure to marijuana is inherently difficult and may be made more difficult by cognitive problems with recollection caused by heavy regular marijuana use. However, we used measurements collected prospectively at the periodic study visits made during the 20-year follow-up in the CARDIA study. At each study visit, participants were asked about current/recent usage (“ . . . in the last 30 days”), and we made the assumption that use between examinations reflected use reported at each examination. While these assumptions introduce measurement error, they do not depend on long-term recollections about total usage over the course of many years. Also, the CARDIA study did not collect information about mode of inhalation (joint, bong, vaporizer, etc), so we could not conduct the subgroup analyses suggested by Patel and Khazeni.