Study limitations are noted. First, withdrawal symptoms were measured via retrospective structured self-report rather than observation. However, the coherence of our findings on the anxiety withdrawal symptoms with earlier studies adds strength to the aggregated literature on this aspect of cannabis withdrawal, while the finding on the weakness factor merits further investigation. Second, the symptoms analyzed included those listed for other substances in DSM-IV. While multiple symptoms of anxiety, physical discomfort, and negative affect were included, irritability was not. As noted above, a number of studies show irritability as a cannabis withdrawal symptom. While our factors might have been altered if we had included irritability, irritability tends to co-occur with anxiety, depression, and insomnia1, 7, 22 suggesting that the addition of irritability would not result in a major restructuring of the factors. However, future epidemiologic studies on cannabis withdrawal should clearly include irritability. Third, we did not have precise measures of the amount of Δ9-THC ingested and cannabis potency and efficiency of smoked self-administration can vary considerably. Most of those in our sample appear to have been heavily exposed to
