BACKGROUND: The use of medications to treat obesity remains controversial. Our goal was to assess national trends in antiobesity medication use with a focus on patterns surrounding the 1997 removal of antiobesity drugs from the market. METHODS: Using a serial cross-sectional study design, we analyzed a nationally representative sample of US office-based physician visits from 1991 to 2002. Data come from the IMS HEALTH National Disease and Therapeutic Index. These data include a sample of 13 452 patient visits for which a diagnosis of clinical obesity was made, with annual visits ranging from 666 in 1994 to 1854 in 1996. The unit of analysis is the patient visit, while the primary outcome measures are the annual and quarterly number of antiobesity drug mentions for clinically obese patients. RESULTS: At its peak in the second quarter of 1997, 2.5 million Americans were taking antiobesity medications, a 4-fold increase over the prior 2 years. Although antiobesity medication use diminished following the market exit of fenfluramine hydrochloride and dexfenfluramine hydrochloride, current levels of use remain above those in the early 1990s. Phentermine has consistently been the most common antiobesity medication. In 2002, an annualized 1.2 million mentions of phentermine use were noted (31% of drug-treated obese patients). Newly released medications, orlistat (0.6 million) and sibutramine hydrochloride (0.4 million), were used less often. Most antiobesity medication use occurs in patients without other reported medical conditions. CONCLUSIONS: Use of antiobesity medications increased rapidly with public and professional interest in fenfluramine-phentermine (fen-phen) combination therapy. Despite reports of adverse outcomes associated with fenfluramine agents (fen-phen and dexfenfluramine), the use of these medication therapies did not diminish until soon before their removal from the market in 1997.