Concurrent use of opioids and benzodiazepines (COB) can lead to additive respiratory and central nervous system effects, putting patients at increased risk of fatal overdose. In 2016, the Centers for Disease Control and Prevention released an opioid-prescribing guideline recommending against COB, and the Pharmacy Quality Alliance (PQA) endorsed a COB measure in its core opioid set. From May 1, 2017, to December 4, 2017, a California Medicaid plan launched a COB-focused prescriber outreach intervention for members receiving recent opioid and benzodiazepine claims with the intent of decreasing concurrent use.To assess the effect of a prescriber fax intervention by a Medicaid plan on COB.Two retrospective analyses were conducted using administrative pharmacy claims data: a comparison of the PQA COB rate among selected California Medicaid plans for 2016 and 2017 and a cohort utilization analysis of members identified for the fax intervention compared with controls. Intervention and control members were matched based on 12 pre-index utilization characteristics. Outcomes assessed included proportion of members with resolution of COB in the post-index period, change in mean number of COB days before and after the index date, and proportion of members with decreased benzodiazepine daily dose after the index date. Analyses were also performed for the subgroups of members with < 30 days of COB and ≥ 30 days of COB in the pre-index period.All California Medicaid plans in the study saw an improvement in the PQA COB rate between 2016 and 2017. In the utilization analysis, 4,182 intervention members were eligible according to study criteria and matched to similar control members. Many differences in medication use existed between the subgroups with < 30 days and ≥ 30 days of COB in the pre-index period, with the latter group consisting of much more chronic, complex users. The intervention cohort had a statistically significant higher proportion of members with complete resolution of COB compared with the control cohort (43.8% vs. 40.0%; < 0.01), which was also statistically significant for the 2 subgroups. The intervention cohort had a decrease in the mean number of COB days from pre- to post-index periods, but this was only statistically significant for the subgroup with < 30 COB days (-2.5 vs. -1.5; = 0.0217). No statistically significant differences were detected between cohorts in proportion of members with decreased benzodiazepine dose.Our analyses demonstrated that this low-touch prescriber fax intervention produced statistically significant improvements in COB outcomes, despite the overall trend of declining COB among the other California Medicaid plans. Low-touch, targeted prescriber outreach can be an inexpensive yet effective tool to affect prescriber behavior, particularly before COB becomes chronic.No outside funding was used to support this study. The authors do not have any financial relationships or potential conflicts of interest relevant to this article to disclose. At the time of conducting this research, all authors were employees of MedImpact Healthcare Systems. The results of this study were presented at the AMCP Managed Care & Specialty Pharmacy Annual Meeting 2019; March 25-28, 2019; San Diego, CA.