TABLE 3

Results from mixed-effects regression models for marijuana smokers and tobacco smokers showing the longitudinal lung function decline (adjusted for pack-years or joint-years and other covariates) shown as rate of change in forced expiratory volume in 1 s (FEV₁)

Predictor variables	Subjects n	Rate of change in FEV₁ mL·year⁻¹
Predictor variables	Subjects n	β-coefficient (95% CI)	Absolute change
Model 1
Never-smokers (reference)	482	−10.75	−10.75
Marijuana smoking joint-years groups
>0–1	56	−7.28 (−17.95–3.40)	−18.03
>1–5	72	−16.51 (−33.18–0.17)	−27.26
>5–20	63	2.12 (−9.55–13.78)	−8.63
>20	51	−29.45 (−46.58– −12.32)*	−40.20
Model 2
Never-smokers (reference)	482	−11.20	−11.20
Tobacco smoking pack-years groups
>0–1	59	2.22 (−15.67–20.10)	−8.98
>1–5	65	1.22 (−9.88–12.32)	−9.98
>5–20	207	−6.78 (−15.37–1.80)	−17.98
>20	439	−21.13 (−27.46– −14.81)*	−32.33
Model 3
Never-smokers (reference)	482	−11.46	−11.46
Heavy marijuana smoking (>20 joint-years)
Current	34	−30.91 (−53.56– −8.27)*	−42.37
Former	17	−27.10 (−51.78– −2.42)*	−38.56
Model 4
Never-smokers (reference)	482	−9.74	−9.74
Heavy tobacco smoking (>20 pack-years)
Current	272	−40.32 (−51.54– −29.11)*	−50.06
Former	167	−8.42 (−14.33– −2.51)*	−18.16

The β-coefficient for each smoking subgroup/category is the mean rate of change of FEV₁ relative to (compared with) the reference (never-smokers of either tobacco or marijuana). The absolute rate of decline for the smoking subgroup is therefore β-coefficient of the subgroup added to the rate of decline of the reference (never-smoker); for example, in model 1, the absolute change for marijuana smokers of >20 joint-years is the sum of −29.45 and −10.75, that is −40.20 mL·year⁻¹. In model 2, the absolute change for tobacco smokers of >20 pack-years is −32.33 mL·year⁻¹, and so on for models 3 and 4. *: significantly different compared with that of never-smokers of either marijuana or tobacco, p<0.05.