Flufenamic acid-d4 is intended for use as an internal standard for the quantification of flufenamic acid by GC- or LC-MS. Flufenamic acid is a non-steroidal anti-inflammatory drug (NSAID) and COX inhibitor (IC50s = 3 and 9.3 µM for human COX-1 and COX-2, respectively). Flufenamic acid inhibits TNF-α-induced increases in COX-2 levels and NF-κB activation in HT-29 colon cancer cells in a concentration-dependent manner. It inhibits calcium influx induced by fMLP or A23187 in human polymorphonuclear leukocytes (PMN) with IC50 values of 29 and 14 µM, respectively. Flufenamic acid also activates various ion channels, including transient receptor potential canonical 6 (TRPC6) and the large-conductance calcium-activated potassium channel (KCa1.1). It also inhibits various ion channels, including TRPC3 and the cystic fibrosis transmembrane conductance regulator (CFTR). Flufenamic acid (20 mg/kg) reduces increases in intestinal fluid secretion and intestinal barrier disruption in mice ......