Every day, millions of Americans face barriers accessing prescription drugs because of delays and denials driven by prior authorization. Though this practice seeks to guard against runaway health care costs and unnecessary treatments, it can keep out of reach quite necessary drugs for common conditions such as diabetes and depression. Prescription drugs are a particularly important setting in which to examine this insurance practice not only because approximately two-thirds of American adults take prescription drugs, but also because this setting is astonishingly complex, placing heavy informational burdens on patients as well as on their physicians. This complexity is driven in part by each health insurer managing its own formulary, such that there is uncertainty about coverage whenever one changes insurers, with physicians often feeling as though they are prescribing in a black box. Drawing on interviews with patients and physicians, this chapter shows that discretion-laden medical necessity guidelines, coupled with formulary variation across insurers, can lead to treatment interruptions that exact a health and economic price for patients, especially those with fewer resources.