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Quantitative assessment of betamethasone in patients with RA and SA.Betamethasone elimination was monitored during continual NSAID therapy.Betamethasone revealed a few cases of bi-exponential elimination.Dual-acting formulation of the drug was detected up to 28 days after administration.Quantitative evaluation and assessment of pharmacokinetic parameters of Diprospan® (suspension for injection 7 mg/mL (2 mg + 5 mg/mL) of betamethasone) were performed in urine samples taken from patients with rheumatoid arthritis or ankylosing spondylitis for 28 days after systemic intramuscular administration in routine clinical practice in an open-comparative prospective cohort study. The maximum betamethasone concentration was reached at day 4 of the follow-up; in some cases, β-phase of elimination of the drug was appeared at day 14 or at day 21 of the follow-up. The deferred β-phase elimination was likely a consequence of the physiological characteristics of the patients or of the influence of non-steroidal agents. The half-life of betamethasone was 8.5 days. The elimination rate constant was 2.49 h-1; the mean clearance was 4.72 L/d. The recommended frequency of the drug administration to its complete elimination was estimated up to 48 days. Mann-Whitney test showed no significant differences in pharmacokinetic characteristics between male and female subjects. The prolonged elimination phase was observed in patients with deviations in their body mass index, continual treatment by diclofenac and nimesulide or, possibly, after consuming an alcohol. The study was recorded in Clinical Trials open source with identifier NCT03119454.