Table 3.

Multivariable analysis of factors associated with heroin–methamphetamine co-injection in the past 6 months among persons who inject drugs in San Diego and Tijuana (n = 1,224)

Variable	Unadjusted bivariate OR [95% CI]	Model 1 (main effects) AOR [95% CI]	Model 2 (interaction) AOR [95% CI]
Age (per-5 year increase)	0.85 [0.80, 0.89]	0.93 [0.88, 0.99]	0.93 [0.88, 0.99]
Distributive syringe sharing	2.44 [1.89, 3.14]	1.88 [1.42, 2.49]	1.87 [1.41, 2.47]
Purchase prefilled syringe	2.08 [1.94, 4.05]	1.64 [1.09, 2.12]	1.66 [1.09, 2.49]
Past-6-month overdose	2.14 [1.45, 3.15]	1.74 [1.13, 2.83]	–
Hard to obtain sterile syringes	1.71 [1.28, 2.78]	1.53 [1.11, 2.14]	1.54 [1.11, 2.13]
Great or urgent need for help for drug use	1.65 [1.32, 2.06]	1.44 [1.12, 1.84]	1.46 [1.12, 1.86]
Tijuana cohort vs. San Diego cohort	5.13 [3.98, 6.61]	4.27 [3.23, 5.65]	–
Past-6-Month Overdose × Cohorta
Tijuana			1.27 [0.76, 2.13]
San Diego			3.05 [1.53, 6.08]

Notes: Reference group is “did not co-inject heroin and methamphetamine in the past 6 months.” OR = unadjusted odds ratio; AOR = adjusted odds ratio; CI = confidence interval.

^ap value for Overdose × Cohort term = .046.