We analysed computerized records of disease-modifying anti-rheumatic drug (DMARD) monotherapy to determine how long rheumatoid arthritis (RA) patients continued on five commonly prescribed DMARDs, and the incidence and time-course of adverse drug reactions (ADRs) they experienced. We studied the records for 3923 courses of DMARDs given to a cohort of 2170 patients monitored for a total of 9378 treatment-years. Methotrexate (MTX) was the DMARD most likely to be continued long-term; <45% of patients had discontinued the drug after 96 months. For the other DMARDs, the time until 50% discontinued due to ADRs or inefficacy was 43.3 months for sulphasalazine (SAS), 33.9 months for D-penicillamine (DPN) and 26 months for myocrisin. Most monitored ADRs requiring drug discontinuation were seen early in therapy, with a median time to onset of <6 months; the important exceptions to this were haematological ADRs to MTX, where the median delay to neutropenia was 16.9 months, and that to thrombocytopenia was 9.4 months. Monitored ADRs (identified by blood or urine tests) were seen least frequently with SAS (one ADR in every 35 patient-years of monitoring) but this apparent advantage was offset by a high incidence of gastrointestinal ADRs and inefficacy. Overall, one toxicity reaction requiring drug discontinuation was identified for every 15.9 patient-years of monitoring.