Methoxyflurane

Methoxyflurane, sold under the brand name Penthrox (the "green whistle") among others, is an inhaled medication primarily used to reduce pain following an injury. It may also be used to reduce pain associated with minor medical procedures. Onset of pain relief is rapid and a standard dose typically lasts for up to 30 minutes. Use is only recommended with direct medical supervision. Common side effects include anxiety, headache, sleepiness, cough, and nausea. Serious side effects may include kidney problems, liver problems, low blood pressure, and severe anaesthetic reactions such as malignant hyperthermia. It may be used during pregnancy or breastfeeding, however there may be additional harmful side effects. It is only recommended in those who have a normal level of consciousness and stable blood pressure and heart rate. It is classified as a volatile anaesthetic. It was first made in 1948 by William T. Miller and came into medical use in the 1960s. It was used as a general anesthetic from its introduction in 1960 until the late 1970s. In 1999, the manufacturer discontinued methoxyflurane in the United States, and in 2005 the Food and Drug Administration withdrew it from the market, due to reports of nephrotoxicity and hepatotoxicity. As of April 2025, it is used in New Zealand, Australia, Canada, Ireland, and the United Kingdom for acute pain.