Market competition levels were associated with a change in generic drug prices. Such measurements may be helpful in identifying older prescription drugs at higher risk for price change in the future. From 1.08 billion prescription claims, a cohort of 1120 generic drugs was identified. After adjustment, drugs with quadropoly (HHI value of 2500, indicating relatively high levels of competition), duopoly (HHI value of 5000), near-monopoly (HHI value of 8000), and monopoly (HHI value of 10 000) levels of baseline competition were associated with price changes. The 5.5 years of data were divided into 11 study periods of 6 months each. The Herfindahl–Hirschman Index (HHI) calculated by summing the squares of individual manufacturers’ market shares, with higher values indicating a less competitive market—and average drug prices were estimated for the generic drugs in each period.

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