Opioid overdose deaths in the United States have climbed since 1999. In 2014, the Affordable Care Act prompted some states to expand Medicaid programs, providing low-cost prescription access to millions of Americans. Some have questioned whether Medicaid expansion might worsen the opioid crisis. To test the association between the expansion of state Medicaid programs and Medicaid-paid prescriptions of opioid pain relievers and opioid addiction therapies. We analyzed the 2010-2016 Medicaid State Drug Utilization Data using a difference-in-differences regression approach, comparing prescriptions per enrollee between states that expanded Medicaid in 2014 and states that did not. We compared opioid pain relievers and opioid addiction therapies to 5 other commonly prescribed drug types important to the Medicaid expansion population (antidepressants, antihypertensives, diabetes medications, cholesterol treatments, and contraceptives) and to overall prescription volume. A secondary analysis compared opioid pain relievers and opioid addiction therapies, between states with high and low overdose death rates. We found overall prescription use per enrollee was higher after 2014. Relative growth in opioid pain reliever prescriptions was modest compared with growth in medications for depression, hypertension, diabetes, and high cholesterol. Growth in prescriptions used to treat opioid use disorder greatly outpaced other drugs, suggesting important gains in access to addiction treatments; growth was higher in states with higher pre-2014 overdose death rates. Our results suggest Medicaid expansion benefited a population with unique needs, and that Medicaid expansion could be a valuable tool in addressing the opioid overdose epidemic.