Objective: The effects of a dose-reduction intervention of biological disease-modifying anti-rheumatic drugs (bDMARDs) in patients in remission were analysed with epidemiology and health economics strategies. The aims were to analyse changes in bDMARD dosage, evaluate potential disease worsening, and estimate cost reduction. Method: This uncontrolled single-centre observational study analysed bDMARD-treated patients with rheumatoid arthritis (RA), psoriatic arthritis (PsA), and spondyloarthritis (SpA). bDMARD expenditure constituted a proxy for bDMARD doses, which enabled group-level analysis. Interrupted time-series regression was used to analyse changes in treatment cost due to the dose reduction. Disease activity and treatment durations were monitored to investigate disease worsening. Results: In total, 997 biological treatment cases were analysed. This involved 527 bDMARD patients, where an unknown fraction of patients was given reduced doses. Disease activity of RA and PsA patients decreased from 2001 to 2009 and remained stable after that, while disease activity for SpA patients was unchanged, indicating no disease worsening from the intervention. The dose tapering resulted in decreased bDMARD expenditure, indicating a decrease in bDMARD consumption, which led to an accumulated cost reduction of 4 178 000 EUR. Conclusions: The results suggest that dose reduction can be safely performed in patients in treatment remission on a group level without compromising treatment efficacy. Subcutaneous bDMARDs, including abatacept, adalimumab, and etanercept, were observed to be well suited to customizing dosage. This study highlights the potential for individualized and personalized rheumatic medicine by providing dose reduction to individual patients, while monitoring disease activity.