AbstractOjeok‐san is a frequently used herbal medication for the management of osteoarthritic pain. We evaluated the effect of Ojeok‐san on the pharmacokinetics of celecoxib at steady‐state in healthy individuals. An open‐label, fixed‐sequence, two‐period, two‐treatment cross‐over study was conducted. In period I, the individuals received celecoxib capsule 200 mg once daily for 4 days. In period II, only Ojeok‐san (14.47 g/pack, three times daily) was administered for 4 days, followed by co‐administration with celecoxib for 4 days. On the fourth (final) day of administration, Ojeok‐san was administered as a single dose. The blood samples for pharmacokinetic evaluation were collected for up to 48 hr after the administration of celecoxib in each study period. Of the 22 enrolled individuals, 20 individuals completed the study. In the presence of Ojeok‐san, the systemic exposure of celecoxib was decreased. The geometric mean ratios ([celecoxib + Ojeok‐san]/celecoxib) and the 90% confidence intervals for the maximum plasma concentration (C_max) and the area under the plasma concentration–time curve during dosing interval (AUC_τ) of celecoxib at steady‐state were 0.725 (0.620–0.848) and 0.885 (0.814–0.962), respectively. The changes in the mean of the C_max and AUC_τ of celecoxib were greater in intermediate metabolizers of cytochrome 2C9 (CYP2C9) than in normal metabolizers. Our results suggested that the C_max and AUC_τ of celecoxib were reduced by Ojeok‐san co‐administration. This finding may be beneficial to determine the required adjustment of celecoxib dosage when co‐administered with Ojeok‐san.