Metamizole sodium is a water-soluble sodium salt of metamizole, a pyrazolone derivative introduced clinically in the early twentieth century. It is used in many countries for moderate-to-severe acute pain, persistent fever, and smooth-muscle spasm, often in combination products with antispasmodics such as pitofenone and fenpiverinium. Its appeal lies in strong analgesic and antipyretic activity with relatively little of the gastrointestinal and renal toxicity associated with many classical nonsteroidal anti-inflammatory drugs (NSAIDs).
Despite this pharmacologic profile, metamizole has one of the most consequential safety histories in modern analgesic medicine. Rare but potentially fatal agranulocytosis—a profound drop in neutrophils—led to withdrawal or non-approval in the United States, Canada, the United Kingdom, Australia, Japan, and several other markets. Nevertheless, it remains widely prescribed or available over the counter in parts of Europe, Latin America, Russia, Israel, China, and other regions, making it a striking example of divergent national benefit-risk judgments.
Chemical Structure
Metamizole sodium has the molecular formula C13H16N3NaO4S and a molecular weight of approximately 351.35 g/mol. Chemically, it is the sodium salt of [(2,3-dihydro-1,5-dimethyl-3-oxo-2-phenyl-1H-pyrazol-4-yl)methylamino]methanesulfonic acid; common synonyms include dipyrone sodium, analgin, noramidopyrine methanesulfonate sodium, and sulpyrine.
The molecule combines a pyrazolone ring—the pharmacophore responsible for its analgesic and antipyretic actions—with a methanesulfonate group and a sodium counterion. The sulfonate group greatly increases aqueous solubility, enabling oral solutions, drops, tablets, suppositories, and injectable formulations. The sodium methanesulfonate portion is not merely a passive salt: in the body, metamizole undergoes nonenzymatic hydrolysis to 4-methylaminoantipyrine (MAA), which is further metabolized to other active pyrazolone metabolites.
Metamizole Sodium-Based Medicines
Common products containing this API include:
- Analgin tablets.
- Baralgin M tablets and injection.
- Novalgin tablets, Novolgin oral drops, and injection.
- Optalgin tablets.
- Metamizol-ratiopharm tablets and injection.
- Spasmalgon tablets, containing metamizole sodium with pitofenone and fenpiverinium.
- Tempalgin tablets, containing metamizole sodium and tempidone in some markets.
- Generic metamizole sodium tablets, oral drops, suppositories, and injection.
Product availability and formulations differ by country. Some products have been discontinued or restricted in particular markets, and combination products should not be assumed equivalent to single-ingredient metamizole sodium.
Mechanism of Action
The exact mechanism of metamizole is not completely established, but current evidence supports a combined central and peripheral analgesic action. Its principal active metabolite, MAA, reduces prostaglandin E2-mediated pain sensitization through effects on cyclooxygenase-dependent arachidonic-acid metabolism. Unlike many traditional NSAIDs, however, metamizole’s clinical profile is not fully explained by COX inhibition alone.
Additional proposed mechanisms include activation of the endogenous cannabinoid system, particularly cannabinoid receptor type 1 signaling by the metabolite 4-aminoantipyrine, opening of ATP-sensitive potassium channels through cGMP-related pathways, and release of endogenous opioid peptides. These actions may account for its effectiveness in visceral pain, colic, postoperative pain, and fever, as well as its mild antispasmodic effect.
Pharmacokinetics
Metamizole is rapidly hydrolyzed after administration to MAA, its main circulating active metabolite. Bioavailability after oral and parenteral administration is described as very similar, and key pharmacokinetic parameters of MAA are nearly identical across these routes. This makes oral therapy effective for many indications, while parenteral forms are reserved for situations requiring rapid relief or when oral intake is not possible.
MAA is further converted to 4-aminoantipyrine (AA) and other metabolites, including 4-formylaminoantipyrine and 4-acetylaminoantipyrine. The active metabolites are distributed widely and undergo hepatic metabolism followed by renal elimination. Because analgesic activity is mediated largely by metabolites rather than the parent compound, metamizole is best understood as a prodrug-like analgesic.
Metamizole-Based Medicines
The following are prominent examples of medicines containing metamizole sodium or closely related metamizole salts. Availability and exact composition vary by country.
| Medicine | Typical metamizole-based composition | Common use |
|---|---|---|
| Analgin | Metamizole sodium | Pain, fever, colic |
| Baralgin / Baralgin M | Metamizole sodium, sometimes with antispasmodics | Visceral and smooth-muscle pain |
| Novalgin | Metamizole sodium / metamizole magnesium | Acute pain and fever |
| Optalgin | Metamizole sodium | Headache, dental pain, fever |
| Spasmalgon | Metamizole sodium with pitofenone and fenpiverinium | Biliary, renal, intestinal, and menstrual colic |
| Tempalgin | Metamizole sodium with tempidone (trimeperidine-related component in some markets) | Moderate-to-severe pain |
| Metamizol-ratiopharm | Metamizole sodium | Pain and fever |
| Santagesik | Metamizole sodium combination product | Pain, fever, and spasm |
These products illustrate the API’s broad formulation flexibility: tablets, oral drops, injectable solutions, and suppositories are used in different markets.
Therapeutic Uses
| Therapeutic area | Typical use of metamizole sodium |
|---|---|
| Acute pain | Postoperative pain, trauma-related pain, dental pain, headache, and musculoskeletal pain |
| Visceral pain | Renal colic, biliary colic, intestinal spasm, dysmenorrhea, and other smooth-muscle pain |
| Fever | Persistent or high fever when other antipyretics are unsuitable or insufficient |
| Cancer-related pain | Adjunctive or stepwise analgesia in some national treatment protocols |
| Combination therapy | With antispasmodics for colicky pain; with other analgesics in selected settings under medical supervision |
Metamizole is generally positioned for moderate-to-severe pain rather than routine minor aches, particularly where its rapid effect and low NSAID-type gastrointestinal risk are considered valuable.
Side Effects
The dominant safety concern is metamizole-induced agranulocytosis, an idiosyncratic, potentially life-threatening reaction in which neutrophils fall below 0.5 × 10⁹/L. It is not reliably dose-dependent, may occur at any point during treatment or shortly after stopping it, and can occur even in patients who previously tolerated the drug. Warning features include fever, chills, sore throat, painful mouth lesions, and signs of infection.
Other important adverse effects include hypersensitivity reactions, urticaria, angioedema, bronchospasm, anaphylaxis, hypotension, and shock, particularly after intravenous administration. Rare hepatic events include acute hepatitis and liver failure.
Drug Interactions
Metamizole can interact with drugs through pharmacokinetic and pharmacodynamic mechanisms. It may reduce ciclosporin exposure, potentially requiring monitoring or dose adjustment, and it has been associated with reduced valproate concentrations. Protein-binding displacement may increase the effect of oral anticoagulants, lithium, methotrexate, oral hypoglycemic agents, and some antihypertensives or diuretics.
The most clinically important interaction is additive myelotoxicity: concomitant use with clozapine, phenytoin, perazine, thiamazole, cytostatics, or other agranulocytosis-inducing drugs may heighten hematologic risk. Enzyme inducers such as barbiturates and phenylbutazone may reduce metamizole effectiveness, while chlorpromazine coadministration can cause severe hypothermia.
Safety Considerations
Patients should be warned to stop metamizole immediately and seek urgent medical assessment for fever, sore throat, mouth ulcers, unexplained infection, rash, facial swelling, breathing difficulty, faintness, or shock-like symptoms. A complete blood count should be obtained promptly when agranulocytosis is suspected; treatment should be discontinued before laboratory confirmation because delay can be dangerous.
Metamizole should be avoided or used only with extreme caution in people with prior metamizole-associated blood dyscrasia, known pyrazolone hypersensitivity, severe hepatic or renal impairment, bone-marrow suppression, or glucose-6-phosphate dehydrogenase deficiency in some national guidance. Intravenous use requires slow administration and close monitoring because rapid injection can cause severe hypotension and shock.
Regulatory Status
Regulatory status is highly fragmented. Metamizole-containing products were withdrawn from the U.S. market in 1977 because of agranulocytosis risk, and the drug is not approved for human use in the United States or Canada. It has also been withdrawn or banned in the United Kingdom, France, Sweden, Norway, Australia, Japan, and India.
In contrast, the European Medicines Agency’s assessments acknowledge that Metamizole sodium remains authorized in numerous EU member states, including Germany, Austria, Belgium, Bulgaria, Croatia, Czechia, Hungary, Italy, Latvia, Lithuania, Luxembourg, the Netherlands, Poland, Portugal, Romania, Slovakia, Slovenia, and Spain.
Clinical Perspective
“Metamizole sodium” remains a valuable but controversial API. Its rapid analgesic, antipyretic, and antispasmodic effects make it useful for selected patients, especially those with visceral pain or contraindications to NSAIDs. However, the rare possibility of fatal agranulocytosis means that its use should be deliberate, time-limited where feasible, patient-specific, and accompanied by clear counseling about warning symptoms.
Plain sodium methanesulfonate, by contrast, is a different substance: a highly water-soluble salt with irritant properties in concentrated form, used chiefly in research and industrial applications rather than as a systemic medicine. Anyone evaluating a product labeled “methanesulfonate sodium” should confirm whether it contains metamizole sodium, another drug’s methanesulfonate salt, or sodium methanesulfonate itself.
