Cyclooxygenase-2 (COX-2) is an enzyme involved in prostaglandin biosynthesis, and plays a key role in the inflammatory response.  In that essential oils, extracted from plants, have been long used for their aromatherapy, analgesic, and antibacterial properties, Hiroyasu Inoue, from Nara Women's University (Japan), and colleagues screened a wide range of commercially available essential oils to assess their anti-inflammatory properties.  They identified six essential oils, namely -- thyme, clove, rose, eucalyptus, fennel and bergamot – that reduced the COX-2 expression in cells by at least 25%. Of these, thyme oil proved the most active, reducing COX-2 levels by almost 75%. Upon further study, the researchers found that carvacrol, a compound present in thyme oil, was the primary active anti-inflammatory agent; when they use pure carvacrol extracts in their tests, COX-2 levels decreased by over 80%.