We sought to estimate size and sources of differences in per capita expenditures on primary care medications in the US versus ten comparable countries combined: Australia, Canada, France, Germany, the Netherlands, New Zealand, Norway, Sweden, Switzerland, and the United Kingdom.Using market research data on year 2015 volumes and sales of medicines, we measure total per capita expenditures on six categories of primary care prescription drugs: hypertension treatments, pain medications, lipid lowing medicines, non-insulin diabetes treatments, gastrointestinal preparations, and antidepressants. We quantified the contributions of five drivers of the observed differences in per capita expenditures.We estimated that the US spent 203% more per capita on primary care pharmaceuticals than did the ten comparable countries. Despite the difference in spending levels, on average, Americans actually purchased 12% fewer days of related therapies than residents of the comparator countries. Most of the observed difference in expenditures was due to higher transaction prices of medicines and the use of a more expensive mix of medicines in the US.If utilization patterns and pharmaceutical prices in the US were similar to those in the 10 comparator countries combined, total spending on primary care pharmaceuticals would fall by 30% or more. Such evidence on the level and drivers of US pharmaceutical expenditures should inform policies in this sector.