Oxymorphone (Opana, Numorphan, Numorphone) or 14-Hydroxydihydromorphinone is a powerful semi-synthetic opioid analgesic first developed in Germany in 1914, patented in the USA by Endo Pharmaceuticals in 1955 and introduced to the United States market in January 1959 and other countries around the same time. It (along with hydromorphone) was designed to have less incidence of side effects than morphine and heroin. It was a success as it differs from morphine and heroin in its effects in that it generates less euphoria, sedation, itching and other histamine effects at equianalgesic doses. This also means a lower dependence liability.

The brand name Numorphan is derived by analogy to the Nucodan name for an oxycodone product (or vice versa) as well as Paramorphan/Paramorfan for dihydromorphine and Paracodin (dihydrocodeine). The only commercially available salt of oxymorphone in most of the world at this time is the hydrochloride, which has a free base conversion ratio of 0.891.

In some countries, hydromorphinol is distributed under the trade names Numorphan and Numorphan Oral. This is a relatively rare exception and the two drugs, whilst both being strong opioid analgesics, are notably different from one another.