Objectives To investigate the influence of early disease-modifying antirheumatic drug (DMARD) treatment on long-term functional outcome in patients with recent-onset inflammatory polyarthritis (IP), and the impact of the duration of first and subsequent DMARD treatment.

Methods 642 subjects from a primary care registry of patients with new-onset IP, recruited 1990–4, were followed up for 10 years. Mean change in Health Assessment Questionnaire (HAQ) scores between baseline and 10 years were compared by time to, and time receiving, first DMARD treatment and total time receiving treatment, using linear regression. Adjustment for time-dependent confounders and censoring was performed using marginal structural weights.

Results When adjusted for baseline and subsequent disease severity, those treated early (<6 months from symptom onset) experienced a non-significant improvement in function compared with those never treated (adjusted mean difference in change (adj_MDIC) in HAQ −0.24; 95% CI −0.58 to 0.09); and a significant benefit for each additional month of treatment within 6 months of the onset of symptoms (adj_MDIC −0.10; 95% CI −0.19 to −0.02). Patients who discontinued their first DMARD within 6 months experienced a significant deterioration in long-term function (adj_MDIC in HAQ 0.28; 95% CI 0.04 to 0.52), while those who continued their first treatment for > 3 years experienced an improvement (adj_MDIC in HAQ −0.37; 95% CI −0.77 to 0.04).

Conclusions The importance of time to, and response to, first DMARD treatment and total duration of DMARD treatment in modifying the 10-year function in patients with IP has been demonstrated.