Products
Metamizole is commercially available as drops, tablets, suppositories, and injectables (Minalgin, Novalgin, Novaminsulfon Sintetica, generics). It has been used medicinally since the 1920s.
Structure and properties
Metamizole (C13H17N3O4S, Mr = 311.4 g/mol) is present in drugs as metamizole sodium. This is the sodium salt and monohydrate of the active ingredient. Metamizole sodium is a white crystalline powder that is very soluble in water. Metamizole structurally belongs to the phenyl pyrazolones. It is a prodrug that is biotransformed in the organism to various active metabolites.
Effects
Metamizole (ATC N02BB02) has analgesic, antipyretic, and antispasmodic (spasmolytic) properties. The mechanism of action is not fully elucidated. Central and peripheral targets are discussed. Possible drug targets include cyclooxygenases and cannabinoid receptors. Metamizole is not classified as a nonsteroidal anti-inflammatory drug (NSAID) but is generally classified as a (nonacidic) analgesic and a pyrazolone.
Indications
For the treatment of severe pain and fever. Metamizole is approved as a second-line agent in many countries. It is often used for colic because of its antispasmodic effects.
Dosage
According to the SmPC. The usual single oral dose for adults with normal renal function is 500 to 1000 mg. The maximum daily dose is 3000 to 4000 mg divided into three to four administrations. For children, dosing is based on body weight.
Contraindications
- Hypersensitivity, including to other pyrazolones or analgesics.
- Impaired bone marrow function or hematopoietic disorders
- Hepatic porphyria
- Glucose-6-phosphate dehydrogenase deficiency
- Parenteral administration in unstable circulation or hypotension.
- Pediatrics, pregnancy: cf.
The full precautions can be found in the drug label.
Interactions
Metamizole may cause a decrease in ciclosporin levels. Chlorpromazine may cause hypothermia when used with metamizole. Furthermore, interactions have been described with oral anticoagulants, captopril, lithium, methotrexate, triamterene, diuretics, and antihypertensives for pyrazolones.
Adverse effects
Possible adverse effects include:
- Hypersensitivity reactions up to and including anaphylaxis (rare).
- Isolated drop in blood pressure, especially with rapid intravenous administration.
- Skin rashes (occasional)
- Blood count disorders (rare to very rare)
- Renal function disorders (very rare)
- Reactions at the injection site
Metamizole is controversial because it can cause blood count abnormalities such as dangerous agranulocytosis. This side effect rarely occurs. Overall, however, metamizole seems to be better tolerated than the more commonly used non-steroidal anti-inflammatory drugs (NSAIDs). Particularly at high doses, urine may become red in color due to the formation of the metamizole metabolite rubazonic acid. This discoloration disappears after treatment.
