BACKGROUND: Transitions from one healthcare setting to another often parallel transitions in health status and can be associated with intentional as well as unintentional changes in patient care. Hospitalization may put patients at increased risk odiscontinuity of medication use. OBJECTIVE: To assess the association between hospitalization and medication therapy discontinuities. METHODS: A retrospective follow-up study was conducted using data obtained from the PHARMO Record Linkage System. We randomly selected patients who had been hospitalized (index date) between July 1, 1998, and June 30, 2000. For each hospitalized patient, one nonhospitalized patient was matched for age, sex, and geographic area, and was assigned the same index date as thecorresponding hospitalized patient. The primary study outcome was the incidence of one or more medication therapy discontinuities at the index date and at several control moments during a period of 18 months before and 18 months after hospital admission. We defined 4 mutually exclusive types of discontinuities: generic-brand substitution, product substitution, therapeutic switch, and stop. RESULTS: The study population comprised 8681 hospitalized patients and an equal number of age/sex-matched nonhospitalized patients. Of all hospitalized patients on drug therapy at the index date (n = 5265) 3322 (63.1%) had one or more medication therapy discontinuities at the index date, compared with 1390 (33.5%) of the nonhospitalized patients taking medication atthe index date (n = 4147; RR 1.82; 95% CI 1.71 to 1.94). The highest risk estimate was found for therapeutic switch (RR 5.34; 95% CI 3.93 to 7.26), followed by product substitution (RR 2.32; 95% CI 1.88 to 2.86) and stop (RR 1.98; 95% CI 1.85 to 2.13).There was no significantly increased risk for generic-brand name substitution (RR 0.87; 95% CI 0.72 to 1.06). CONCLUSIONS: Hospitalization is associated with discontinuity of drugs used in the community setting. Medication stops were observed most frequently. Hospital safety programs should focus attention on medication therapy discontinuities at times of transition to ensure continuity of care in relation to drug therapy.