Spending on World Health Organization essential medicines in Medicare Part D, 2011-15: retrospective cost analysis.

Spending associated with essential medicines grew substantially from 2011 to 2015, driven largely by the increased use of two expensive novel drugs used in treating hepatitis C. Approximately 22% of increased total spending during this period can be attributed to increases in per unit cost of existing drugs. These trends may limit patients' access to essential drugs while also increasing healthcare system costs.