Aminophenazone is a phenyl-pyrazolone derivative with potent analgesic and antipyretic properties. Aminophenazone has been used as salt or complexes, including topically as the salicylate. It was recommended for the treatment of a fever, neuralgia, myositis, acute rheumatism, arthritis, chorea. In 1999 the FDA suspended aminophenazone. The drug caused agranulocytosis. Some of the cases of agranulocytosis were fatal. Another reason for suspending this drug from the market was its ability to react with nitrite-containing food, thus forming carcinogenic nitrosamines. A breath test with 13C-labeled aminopyrine has been used as a non-invasive measure of cytochrome P-450 metabolic activity in liver function tests.