|Systematic (IUPAC) name|
|Sodium [(2,3-dihydro- 1,5-dimethyl- 3-oxo- 2-phenyl- 1H- pyrazol- 4-yl) methylamino] methanesulfonate|
|CAS number||(sodium salt)|
|Mol. mass||311.358 g/mol|
|Half life||1 - 4 hours|
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Metamizole sodium is a non-steroidal anti-inflammatory drug (NSAID), commonly used in many countries as a powerful analgesic and antipyretic. It is better known under the names Dipyrone, Analgin, Novalgin, Algocalmin, and Melubrin.
Metamizole was first synthesized by the German company Hoechst AG (now part of Sanofi-Aventis) in 1920, and its mass production started in 1922. It remained freely available worldwide until the 1970s, when it was discovered that the drug carries a small risk of causing agranulocytosis - a potentially fatal condition. Controversy remains regarding the level of risk. Several national medical authorities have banned metamizole either totally or have restricted it to be available only on prescription, while others have maintained its availability over the counter.
According to comments by Dr Anthony Wong of the University of São Paulo, Brazil in a WHO newsletter, recent studies estimate that the incidence rate of metamizole-induced agranulocytosis is between 0.2 and 2 cases per million person days of use, with approximately 7% of all cases fatal (provided that all patients have access to urgent medical care). In other words, one should expect 50 to 500 deaths annually due to metamizole in a country of 300 million, assuming that every citizen takes the drug once a month. This is not a very high rate compared to other drugs - for example, the prescription drug clozapine is known to be at least 50 times more likely to trigger agranulocytosis. However, at the time the risk was assumed to be much greater and, as such, excessive for an over-the-counter analgesic, especially considering the existence of safer alternatives (e.g., aspirin (ASA) and ibuprofen).
In 1998, Andrade et al. conducted a meta-analysis to compare epidemiologic studies from 1975 to 1995 and estimated that the excess mortality per million from community acquired cases of agranulocytosis, aplastic anaemia, anaphylaxis and serious upper gastrointestinal complications was 592 for diclofenac, 185 for ASA, 25 for metamizole, and 20 for paracetamol. CIOMS IV in the same year reported the excess mortality risk for the same conditions to be: diclofenac = 5.92; ASA = 2.03; metamizole = 0.20; and paracetamol=0.25. These studies certainly suggest that the risks from adverse reactions to metamizole are similar to those posed by paracetamol, a drug widely reputed to be safe. According to the CIOMS IV conclusion “Newer methods of epidemiological studies have shown that the risk of agranulocytosis (1.7 per million) due to metamizole was exaggerated in the 70’s”.
A study in Northern Sweden published in 2002 estimated the total risk during metamizole therapy for patients in hospitals (inpatients) and outside of hospital (outpatients) about 3 to 100 times greater than that estimated by Dr Wong: "Given certain assumptions including the actual amounts prescribed the calculated risks of agranulocytosis would be approximately one out of every 31,000 metamizole-treated inpatients and one of every 1400 metamizole-treated outpatients."
Metamizole was banned in Sweden in 1974, in the United States in 1977. Since then, more than 30 countries (including Japan, Australia, Iran, and several of the European Union member nations) have followed suit. In these countries, metamizole is still occasionally used as a veterinary drug. In Germany, Hungary, Portugal and Spain it is a prescription drug. Some European pharmaceutical companies, notably Hoechst and Merck, continue to develop metamizole-containing drugs and market them in some countries. In Sweden, the ban was lifted in 1995 only to be re-introduced in 1999.
In other parts of the world (including Bulgaria, Mexico, India, Egypt, Brazil, Poland, Russia, Turkey, the Republic of Macedonia, Romania, Israel, Kosovo and some developing countries) metamizole is still freely available over-the-counter, remains one of the most popular analgesics, and plays an important role in self-medication. For example, metamizole and metamizole-containing drugs account for 80% of OTC analgesic market in Russia, whereas ibuprofen accounts for 2.5%. In Brazil, metamizole (Novalgina) products, although over-the-counter, carry warnings to avoid usage by those under 19 years old, and include some information about early detection and treatment of agranulocytosis. Although the Brazilian government did not push for a ban on the drug, its use has seen a decline in the past years as pharmaceutical companies and doctors pushed aspirin, paracetamol and ibuprofen based products as replacement, especially regarding child care. Amongst adults it is still widely used. Some of the most widely available metamizole-containing products still in use in Brazil are: Buscopan Plus (under the name of Buscopan Composto), Novalgina and Neosaldina. Generic Dipyrone is also available.
Metamizole has such high usage in Brazil, that in late 2008 Sanofi-aventis released a new version of the product with 1g of metamizole per tablet (twice the normal 500 mg dosage available before), still under the Novalgina name.
A randomized, double blind, multinational study involving 555 children showed that metamizole and ibuprofen were significantly more effective than paracetamol in achieving normal body temperatures; metamizole produced a significantly greater temperature reduction than ibuprofen and paracetamol, and helped maintain low temperatures for a longer duration.
Metamizole received a brief period of attention by American media in 2001, when a Latino immigrant boy was admitted into a Salt Lake City clinic with symptoms of agranulocytosis. It was discovered that the drug remained freely available in Latino shops and highly popular among Mexican immigrants, despite the ban. The ongoing "LATIN" Study, a multicenter international case-control study, is examining the incidence of agranulocytosis in Latin America and the role of metamizole.