By influencing the size and bargaining power of private insurers, public subsidization of private health insurance may project effects beyond the subsidized population. We test for such spillovers by analyzing how increases in insurer size resulting from the implementation of Medicare Part D affected drug prices negotiated in the non-Medicare commercial market. On average, Part D lowered prices for commercial enrollees by 3.7 percentage. The external commercial market savings amount to $1.5 billion per year, which, if passed to consumers, approximates the internal cost savings of newly insured subsidized beneficiaries. If retained by insurers, it corresponds to a greater than 9.25 percentage average increase in profitability on stand-alone drug insurance.

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