TABLE 4.

Relative Risk Ratio for Misuse of Prescription Opioids in Follow-ups 1 to 3 for Respondents With Versus Without Legitimate Prescription Opioid Use by 12th Grade, by Risk Strata (95% Confidence Intervals in Parentheses)

Strata No.	1	2	3	4	5	6	7	8
Predicted probability of opioid misuse after high school	0%–<1.75%	1.75%–<3%	3%–<5%	5%–<10%	10%–<20%	20%–<30%	30%–<45%	≥45%
Legitimate prescription opioid use by 12th grade	0.97 (0.22–4.29)	3.01** (1.79 –5.07)	1.95* (1.15–3.34)	1.24 (0.81–1.91)	0.98 (0.66–1.44)	1.06 (0.64–1.73)	0.56 (0.31–1.01)	0.86 (0.49–1.63)

The risk ratios in stratums 2 and 3 are significantly higher than those in the other stratums combined, as tested with multiplicative interaction terms. Test models included (a) all the predictors in model 2 of Table 2, (b) an indicator variable for risk stratum, and (c) a multiplicative interaction term of the risk stratum indicator and legitimate opioid prescription by 12th grade. In separate models, interactions were statistically significant (P < .05) when including an indicator for risk stratum 2, risk stratum 3, and an indicator for risk strata 2 and 3 combined. *P < .05; **P < .01.