Table 1.

IQ before and after cannabis use

N% maleAge 7–13 full-scale IQAge 38 full-scale IQΔ IQ effect size*
Persistence of cannabis dependence
 Never used, never diagnosed24238.8499.84 (14.39)100.64 (15.25)0.05
 Used, never diagnosed47949.48102.32 (13.34)101.25 (14.70)−0.07
 1 diagnosis8070.0096.40 (14.31)94.78 (14.54)−0.11
 2 diagnoses3562.86102.14 (17.08)99.67 (16.11)−0.17
 3+ diagnoses3881.5899.68 (13.53)93.93 (13.32)−0.38
Persistence of regular cannabis use
 Never used24238.8499.84 (14.39)100.64 (15.25)0.05
 Used, never regularly50850.59102.27 (13.59)101.24 (14.81)−0.07
 Used regularly at 1 wave4772.34101.42 (14.41)98.45 (14.89)−0.20
 Used regularly at 2 waves3663.8995.28 (10.74)93.26 (11.44)−0.13
 Used regularly at 3+ waves4178.0596.00 (16.06)90.77 (13.88)−0.35
  • Means (SDs) are presented for child and adult full-scale IQ as a function of the number of study waves between ages 18 y and 38 y for which study members met criteria for cannabis dependence or reported using cannabis on a regular basis (at least 4 d/wk). The last column shows that study members with more persistent cannabis use showed greater IQ decline from childhood to adulthood.

  • *This coefficient indicates change in IQ from childhood to adulthood, with negative values indicating decreases in IQ. These change scores are in SD units, with values of 0.20, 0.50, and 0.80 reflecting small, medium, and large changes, respectively.