This paper makes two contributions to the literature.  We document the first evidence of causal spillover effects of health behavior choices between spouses by leveraging experimental data within RCT settings. Second, we discuss the implications of social spillovers for program evaluation in health settings; current methods of cost effectiveness and cost benefit analysis implicitly assume zero spillovers among social ties. This assumption can both underestimate the benefits of health interventions and misallocate resources toward interventions with lower comprehensive (direct + indirect) benefits due to the lack of incorporating indirect benefits. Specifically, we use data from the Lung Health Study (LHS) and COMBINE Study to estimate spillovers on smoking and drinking, respectively. We find large reductions, about 50 percent or more of the magnitude of treatment effects on subjects, in spousal substance use in both studies from treatments with a therapy component but no evidence of spillovers in pharmaceutical-only treatments. Our comprehensive cost-effectiveness analysis finds that incorporating benefits from spousal spillovers decreases the incremental cost effectiveness ratios of LHS treatments by 13.8 to 15.5 percent and the COMBINE therapy treatment by 9.6 percent. Overall, our findings suggest that some interventions do have important spillovers onto subject peers that should be accounted for in data collection and analysis.