Non-steroidal anti-inflammatory drugs (NSAIDs) are potent inhibitors of both COX-1 and COX-2 in humans, but are frequently supplied as racemates. Such is the case for the common 2-aryl propionic acid drug flurbiprofen, sold under the trade name Ansaid. In the 2-aryl propionic acid NSAID family, cyclooxygenase inhibition resides primarily in the (S)-enantiomer. In guinea pig whole blood, the IC50 value of (S)-flurbiprofen is about 0.5 µM for both COX-1 and COX-2. Only a small amount (<5%) of the (S)-enantiomer is converted to the (R)-enantiomer in rats and humans; therefore, the biological effects are specific to each enantiomer.