OBJECTIVES: FDA-issued safety alerts and warnings play a vital role in post-market drug surveillance.  We examine the effects of the FDA safety announcements for rosiglitazone and pioglitazone on the utilization of these drugs and other oral antidiabetics. METHODS: We link safety announcements from the FDA MedWatch database to Medicare drug claims (Parts D & B) from 2006 to 2010. We examine the timing, direction and level of demand responses to safety announcements, and how they differ by patients’ race/ethnicity, socioeconomic status and plan type (PDP vs MA-PD). We then estimate the fraction of patients that continue using an “alerted” medication, switch to another oral antidiabetic, or stop use altogether (without adding another oral medication), and how these responses differ by patient group and plan type. RESULTS: The demand for rosiglitazone plummeted after the second safety alert (in May 2007), and decreased further after a (black box) warning was issued (in August).  After the alert, 27% to 28% of rosiglitazone users switched to another oral antidiabetic (predominantly pioglitazone) within 6 months of the alert depending on patient groups and plan type; 28% to 33% discontinued use of rosiglitazone, but did not add another oral antidiabetic; 38% to 44% continued using rosiglitazone. After the warning, the numbers were 17% to 19%, 30% to 32% and 49% to 52% respectively. Discontinuation rates were slightly higher among Hispanics (32%) and those in MA-PD plans (32%). In contrast, the demand response for pioglitazone was more muted, while it increased slightly after the warning (in August). CONCLUSIONS: The demand response to safety warnings for rosiglitazone was large and abrupt, and a substantial fraction of those who stopped did not replace it with another oral antidiabetic. The demand response was fairly constant across race/ethnicity, socioeconomic status, and plan type, although large responses raise concern about adverse health consequences.