Prescription drug coverage creates a change in medical care consumption, beyond standard moral hazard, arising both from the differential cost-sharing and the relative effectiveness of different types of care. We model the dynamic supplemental health insurance decisions of Medicare beneficiaries, their medical care demand, and subsequent health outcomes over time. Using parameter estimates obtained with longitudinal individual-level data, we simulate behavior under different drug coverage scenarios. Prescription drug coverage increases drug expenditures by 7 percent to 27 percent over a five-year period. While mortality rates fall slightly, the survivors have poorer health, leading to higher total medical expenditures. (Contains 21 tables, 4 figures and 19 footnotes.)