IMPORTANCE Many Medicare beneficiaries undergo more intensive colonoscopy screening than recommended. Whether this is favorable for beneficiaries and efficient from a societal perspective is uncertain. OBJECTIVE To determine whether more intensive colonoscopy screening than recommended is favorable for Medicare beneficiaries (ie, whether it results in a net health benefit) and whether it is efficient from a societal perspective (ie, whether the net health benefit justifies the additional resources required). DESIGN, SETTING, AND PARTICIPANTS Microsimulation modeling study of 65-year-old Medicare beneficiaries at average risk for colorectal cancer (CRC) and with an average life expectancy who underwent a screening colonoscopy at 55 years with negative results. INTERVENTIONS Colonoscopy screening as recommended by guidelines (ie, at 65 and 75 years) vs scenarios with a shorter screening interval (5 or 3 instead of 10 years) or in which screening was continued to 85 or 95 years. MAIN OUTCOMES AND MEASURES Quality-adjusted life-years (QALYs) gained (measure of net health benefit); additional colonoscopies required per additional QALY gained and additional costs per additional QALY gained (measures of efficiency). RESULTS Screening previously screened Medicare beneficiaries more intensively than recommended resulted in only small increases in CRC deaths prevented and life-years gained. In comparison, the increases in colonoscopies performed and colonoscopy-related complications experienced were large. As a result, all scenarios of more intensive screening than recommended resulted in a loss of QALYs, rather than a gain (ie, a net harm). The only exception was shortening the screening interval from 10 to 5 years, which resulted in 0.7 QALYs gained per 1000 beneficiaries. However, this scenario was inefficient because it required no less than 909 additional colonoscopies and an additional $711 000 per additional QALY gained. Results in previously unscreened beneficiaries were slightly less unfavorable, but conclusions were identical. CONCLUSIONS AND RELEVANCE Screening Medicare beneficiaries more intensively than recommended is not only inefficient from a societal perspective; often it is also unfavorable for those being screened. This study provides evidence and a clear rationale for clinicians and policy makers to actively discourage this practice.