IQ before and after cannabis use
| N | % male | Age 7–13 full-scale IQ | Age 38 full-scale IQ | Δ IQ effect size* | |
| Persistence of cannabis dependence | |||||
| Never used, never diagnosed | 242 | 38.84 | 99.84 (14.39) | 100.64 (15.25) | 0.05 |
| Used, never diagnosed | 479 | 49.48 | 102.32 (13.34) | 101.25 (14.70) | −0.07 |
| 1 diagnosis | 80 | 70.00 | 96.40 (14.31) | 94.78 (14.54) | −0.11 |
| 2 diagnoses | 35 | 62.86 | 102.14 (17.08) | 99.67 (16.11) | −0.17 |
| 3+ diagnoses | 38 | 81.58 | 99.68 (13.53) | 93.93 (13.32) | −0.38 |
| Persistence of regular cannabis use | |||||
| Never used | 242 | 38.84 | 99.84 (14.39) | 100.64 (15.25) | 0.05 |
| Used, never regularly | 508 | 50.59 | 102.27 (13.59) | 101.24 (14.81) | −0.07 |
| Used regularly at 1 wave | 47 | 72.34 | 101.42 (14.41) | 98.45 (14.89) | −0.20 |
| Used regularly at 2 waves | 36 | 63.89 | 95.28 (10.74) | 93.26 (11.44) | −0.13 |
| Used regularly at 3+ waves | 41 | 78.05 | 96.00 (16.06) | 90.77 (13.88) | −0.35 |
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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.
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↵*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.
