BACKGROUND: In 1996, the US Food and Drug Administration (FDA) approved switching nicotine gum and patch from prescription to over-the-counter (OTC) status. Some expressed concerns that broader availability and lack of physician control might increase persistent use of nicotine replacement therapy (NRT)-that is, use beyond the period specified by the FDA approved label. OBJECTIVE: To estimate the incidence of persistent use of OTC nicotine gum and patch for periods of > 3 months, > or = 6 months, > or = 12 months, and > or 24 months. DESIGN: Analysis of NRT purchase patterns in data from a population based panel of US households that electronically scanned all household purchases between January 1997 and March 2000. SUBJECTS: In a national panel of 40,000 US households, 2690 recorded NRT purchases. RESULTS: Among 805 households that purchased nicotine gum, 2.3% of new purchase incidents led to continuous monthly purchase of gum for > or = 6 months. For nicotine patches (2050 households) the percentage was 0.9%. For both gum and patch, the incidence of persistent purchase dropped below 0.4% by 24 months. Allowing one month gaps within a "continuous" purchase run resulted in increased estimates (for gum: 6.7% for > or = 6 months and 1.0% for > or = 24 months; for patch: 1.7% for > or = 6 months and 0.05% for > or = 24 months). CONCLUSION: Persistent use of nicotine gum and patch is very rare and has not increased with the transition to OTC use, despite removal of physician oversight.