PGP and rheumatoid arthritis: Previous studies reported controversial results regarding the effect of MTX on P-gp expression as MTX downregulated P-gp in the liver (Mohamed et al., 2021), while upregulating it in fibroblast-like synoviocytes of rheumatoid arthritis patients (Qin et al., 2018), and it had no effect on the testis (Morsy et al., 2020a).