A Simple and Sensitive LC‐MS/MS Method for Quantification of Capsaicin in Human Plasma and Its Application to a Clinical Pharmacokinetic Study

column (2.1 × 50 mm, 3.5 μm) with a 3-min gradient elution using 1% formic acid in water and acetonitrile. Detection was conducted on a triple quadrupole tandem mass spectrometer in positive electrospray ionization (ESI) mode, utilizing multiple-reaction monitoring (MRM). The assay was validated over the linear range of 0.05-50 ng/mL. The method demonstrated excellent precision (CV% ≤ 11.62%) and accuracy (RE% within ±13.60%), with high recovery (>95%). Stability was confirmed under various conditions relevant to clinical sample handling. The validated method was successfully applied to a pharmacokinetic study involving 24 patients with knee osteoarthritis receiving a topical capsaicin liniment, demonstrating its suitability for clinical applications.