This dissertation consists of three essays in health economics. The three chapters focus specifically on prescription drug use and treatment in various national and state settings and evaluate the impact of government policies and interventions on this sector of the health care market. The first two chapters focus on opioid prescribing in the United States. Prescription drug monitoring programs (PDMPs or PMPs)—online systems that health care providers and pharmacists can use to query patient prescription records—are one of the most widely-used state tools in regulating the prescribing and dispensing of opioids. However, the staggered adoption of PDMPs over time has created opportunities for patients to evade monitoring by going to a state that does not have a PDMP. Chapter 1 evaluates how spillovers attributable to policy non-coordination between neighboring states impact the effectiveness of PDMPs. I find that after prescribers gain access to PDMPs, opioid volume and prescription opioid deaths decrease in counties with a PDMP that are insulated from opportunities for evasion. I find a similar effect in counties with a PDMP that are exposed to evasion. This suggests that exposure to evasion through proximity to non-PDMP areas does not significantly attenuate the policy effect. I also find evidence that opioid volume and prescription opioid deaths decrease in counties without a PDMP that are exposed to spillovers from counties with the policy. Illicit opioid deaths are not affected in any counties with a PDMP but decrease in counties without a PDMP that are exposed to spillovers. I discuss the potential mechanisms through which spillovers may operate. Chapter 2, which is joint work with Adam Sacarny, David Powell, Ian Williamson, Weston Merrick, and Mireille Jacobson, evaluates how "nudge" interventions can impact the behavior of clinicians prescribing controlled substances. PMPs aim to reduce inappropriate opioid prescribing but may be underutilized by prescribers. We conduct a randomized clinical trial of 12,000 clinicians in Minnesota to test whether letters to providers can increase PMP use and decrease potentially dangerous opioid co-prescriptions. In this study, we focus on the co-prescribing of opioids and benzodiazepines and the co-prescribing of opioids and gabapentinoids. We find that letters that mention the state's new PMP use mandate increase PMP search rates and the share of clinicians with PMP accounts but have no significant effect on co-prescribing. Letters with only information about the risks of co-prescribing and a list of co-prescribed patients have no detected effect on primary outcomes of interest. We also explore the impact of the letters on additional search and prescribing outcomes. Our results highlight the potential for simple letter-based interventions to encourage engagement with PMPs and facilitate better-informed prescribing of opioids and other medications. Finally, Chapter 3 studies the prescription drug market in Japan, examining how changes in health care prices faced by patients can influence demand. I exploit a feature of the Japanese healthcare system, where an individual's coinsurance rate is determined primarily by their age, to evaluate the impact of a change in patient cost sharing on total prescription drug spending. I contribute to the existing literature by investigating heterogeneous effects by patient sex and drug therapeutic class (focusing on cardiovascular drugs, antibiotics, vitamins, antihistamines, and psychotropic drugs). I find that for the whole sample, price elasticity of spending for prescription drugs is comparable to previous estimates of price elasticity of spending for general medical services. I find no evidence of heterogeneous effects by sex over the whole sample of prescriptions, but I do find statistically significant differences between women and men within therapeutic drug classes. I also conduct exploratory analysis on the effect of changes in patient cost sharing on prescription drug volume. I estimate a price elasticity of demand for prescription drugs that is larger than previous estimates of demand elasticity for general medical services. I also find evidence that physicians do not respond on the intensive margin by prescribing more expensive medications. Although Japanese patients are more likely to be prescribed brand-name drugs, patients using generic medications may be more price sensitive to changes in patient cost sharing.