We study Any Willing Provider (AWP) regulations, which limit the ability of insurance companies to provide incentives for the insured to visit particular providers. We provide a model showing how AWP could increase healthcare prices. We then use claims-level insurance data to provide a detailed case study of an AWP regulation that was enacted in Maine in July, 2010. The regulation forced insurers to reimburse any willing pharmacy at pre-announced terms, thus limiting the insurer/pharmacy benefit manager’s ability to commit to a narrow network when negotiating reimbursements. We use self-insured plans and the experience of neighboring states to control for time-varying state effects and time-varying insurance effects in difference in difference in differences estimates of the impact of the regulation on drug prices. We estimate large increases in the price of drugs and drug spending as a result of the regulation.