Benoxaprofen, also known as benoxaphen, is a chemical compound with the formula C₁₆H₁₂ClNO₃. It is a non-steroidal anti-inflammatory drug (NSAID) of the arylpropionic acid class, and was marketed under the brand name Opren in the United Kingdom and Europe by Eli Lilly and Company (commonly referred to as Lilly), and as Oraflex in the United States of America (USA). Lilly suspended sales of Oraflex in 1982 after reports from the British government and the United States Food and Drug Administration (US FDA) of adverse effects and deaths linked to the drug.

Benoxaprofen was discovered by a team of research chemists at the British Lilly Research Centre of Eli Lilly and Company . This laboratory was assigned to explore new anti-arthritic compounds in 1966. Lilly applied for patents on its then named new drug 'benoxaprofen' seven years later. It also filed for permission from the U.S. Food and Drug Administration to start testing benoxaprofen on humans. It had to undergo the three-step clinical testing procedure required by the United States Federal Government.

Lilly began Phase I of the benoxaprofen clinical trials by testing a selection of healthy human volunteers. These tests had to prove that their new drug posed no clear and immediate safety hazards. In Phase II, a larger number of human subjects, including some with minor illnesses, was tested; the drug's effectiveness and safety was the major target of these tests. Phase III was the largest test, and began in 1976. More than 2,000 arthritis patients were administered benoxaprofen by more than 100 physicians. The physicians then reported the results to the Lilly Company.

When Lilly formally requested to begin marketing benoxaprofen in January 1980 with the US FDA, the document consisted of more than 100,000 pages of test results and patients records. However, benoxaprofen was first marketed abroad;in 1980, it was released for marketing in the United Kingdom. It subsequently came on the market in May 1982 in the USA.

When benoxaprofen was on the market as Oraflex in the USA, the first sign of trouble came for the Lilly Company. The British Medical Journal reported in May 1982 that physicians in the United Kingdom believed that the drug was responsible for at least twelve deaths, mainly caused by kidney and liver failure. A petition was filed to have Oraflex removed from the market.

On 4 August 1982, the British government temporarily suspended sales of the drug in UK 'on grounds of safety'. The British Committee on the Safety of Medicines declared, in a telegram to the FDA, that it had received reports of more than 3,500 adverse side effects among patients who had used Oraflex. There were also 61 deaths, most of which were of elderly people. Almost simultaneously, the FDA said it had reports of 11 deaths in the USA among Oraflex users, most of which were caused by kidney and liver damage. The Eli Lilly Company suspended sales of benoxaprofen that afternoon.

The molecular formula of benoxaprofen is C₁₆H₁₂ClNO₃ and the systematic (IUPAC) name is 2-[2-(4-chlorophenyl)-1,3-benzoxazol-5-yl]propionic acid. The molecule has a molecular mass of 301.050568 g/mol.

Benoxaprofen is essentially a planar molecule. This is due to the co-planarity of the benzoxazole and phenyl rings, but the molecule also has a non-planar side chain consisting of the propanoic acid moiety which acts as a carrier group. These findings were obtained from X-ray crystallographic measurements made at the Lilly Research Centre.