Restrictive drug formularies may influence physician prescribing behavior for other patients with more generous drug benefit, so-called “spillover effects.” We focus on Protonix, a proton pump inhibitor, introduced at a discount price, and study subsequent physician prescribing decision for open-formulary Medicaid patients. Using two national databases on physician prescribing patterns and health plan drug formularies, we find consistent evidence of significant, positive spillover effects from 1-PPI and 2-PPI formularies that include Protonix onto Medicaid, with a larger effect from the most restrictive, 1-PPI formularies. Physicians who prescribe a higher proportion of Protonix to their non-Medicaid patients because of restrictive formularies, also prescribe a higher proportion of Protonix to their Medicaid patients with an open formulary. Each 10% increase in Protonix's predicted non-Medicaid share results in an 8% increase in its share of Medicaid prescriptions (p < 0.001). Similar spillover effects seem to exist for older PPI products as well. A restrictive drug formulary therefore influences physician prescribing behavior for other patients not directly subject to its restrictions.