Around 25% of patients with systemic lupus erythematosus (SLE) could be refractory to conventional therapies. P-glycoprotein expression on cell surface has been implied on drug resistance, however, to date, it is unknown if P-gp serum levels are associated with SLE disease activity. Evaluate the association of serum P-gp levels and SLE with disease activity despite treatment. A cross-sectional study was conducted on 93 female SLE patients, all receiving glucocorticoids at stable doses for the previous 6 months before to baseline. SLE patients were classified into two groups: (a) patients with active disease [SLE disease activity index (SLEDAI) ≥ 3] despite treatment, and (b) patients with inactive disease (SLEDAI