|Systematic (IUPAC) name|
|Trade names||Ponstel, Ponstan|
|Excretion||Urine (66%), faeces (20-25%)|
|(what is this?)|
Mefenamic acid is a non-steroidal anti-inflammatory drug used to treat pain, including menstrual pain. It is typically prescribed for oral administration. Its name is a contraction of its chemical name, 2-(2,3-dimethylphenyl)aminobenzoic acid. Mefenamic acid is marketed in the USA as Ponstel and is commonly known in Canada and UK as Ponstan. Ponstan is distributed by ERFA Canada 2012 Inc.
Mefenamic acid decreases inflammation (swelling) and uterine contractions by a still-unknown mechanism. However it is thought to be related to the inhibition of prostaglandin synthesis. There is also evidence that supports the use of mefenamic acid for perimenstrual migraine headache prophylaxis, with treatment starting 2 days prior to the onset of flow or 1 day prior to the expected onset of the headache and continuing for the duration of menstruation.
Since hepatic metabolism plays a significant role in mefenamic acid elimination, patients with known liver deficiency may be prescribed lower doses. Kidney deficiency may also cause accumulation of the drug and its metabolites in the excretory system. Therefore patients suffering from renal conditions should not be prescribed mefenamic acid.
Tablets 100 mg, 250 mg, 500 mg. Capsules: 250 mg. Suspension: 50 mg/5 ml
In the USA, wholesale price of a week's supply of generic mefenamic acid has been quoted as $426.90 in 2014. Brand-name Ponstel is $571.70. In contrast, in the UK, a weeks supply is £1.66, or £8.17 for branded Ponstan. In the Philippines, 10 tablets of 500 mg generic mefenamic acid cost PHP39.00 (or the equivalent of $0.88USD)as of October 25, 2014.
Mechanism of action
Mefenamic acid is a competitive inhibitor of COX-1 and COX-2, which are responsible for the first committed step in prostaglandin biosynthesis. Decreasing the activity of these enzymes thus reduces the production of prostaglandins, which are implicated in inflammation and pain processes.
Absorption: Rapidly absorbed. Tmax is 2 to 4 h. Cmax is 20 µg/mL (single doses). Steady state is reached in 2 days.
Distribution: Apparent Vd is 1.06 L/kg. Protein binding is more than 90%. Excreted in breast milk.
Metabolism: Metabolized by CYP-450 enzyme CYP2C9 to 3-hydroxymethyl mefenamic acid (metabolite I). Further oxidation to a 3-carboxymefenamic acid (metabolite II) may occur. The metabolites may undergo glucuronidation, and mefenamic acid is also glucuronidated directly.
Elimination: Approximately 52% of a dose is excreted into the urine primarily as glucuronides of mefenamic acid (6%), 3-hydroxymefenamic acid (25%), and 3-carboxymefenamic acid (21%). The fecal route of elimination accounts for up to 20% of the dose, mainly in the form of unconjugated 3-carboxymefenamic acid. The half-life is approximately 2 h.
- Cardiovascular: CHF; hypertension; syncope; tachycardia.
- CNS: Dizziness, headache (up to 10%).
- Dermatologic: Pruritus, rashes (up to 10%).
- GI: Abdominal pain, constipation, diarrhea, dyspepsia, flatulence, GI ulcers (gastric/duodenal), gross bleeding/perforation, heartburn, nausea, vomiting (up to 10%).
- Hematologic: Anemia, increased bleeding time (up to 10%).
- Hepatic: Elevated liver enzymes (up to 10%).
- Miscellaneous: Abnormal kidney function, edema, tinnitus (up to 10%).
Known mild side effects of mefenamic acid include headaches, nervousness and vomiting. Serious side effects may include diarrhea, hematemesis (vomiting blood), haematuria (blood in urine), blurred vision, skin rash, itching and swelling, sore throat and fever. It is advised to consult a doctor immediately if these symptoms appear while taking this medication.
Mefenamic acid is recommended to be taken with food.
Overdose can lead to a range of symptoms including convulsions, nausea, vomiting, hematemesis, slowed rate of breathing, and coma. Onset of symptoms is usually between 30 minutes and 4 hours, but signs of kidney failure may appear several days after an overdose. Seek medical attention immediately in the case of overdose. The lethal dose can be as low as 2.5 g.
Mefenamic acid cannot be expected to substitute for corticosteroids or to treat corticosteroid insufficiency. Abrupt discontinuation of corticosteroids may lead to disease exacerbation. Patients on prolonged corticosteroid therapy should have their therapy tapered slowly if a decision is made to discontinue corticosteroids. The pharmacological activity of mefenamic acid in reducing fever and inflammation may diminish the utility of these diagnostic signs in detecting complications of presumed noninfectious, painful conditions. People with kidney and liver impairment or with asthma. Monitor blood counts and liver function during long-term therapy.
- Pringsheim, T.; Davenport, W. J.; Dodick, D. (2008). "Acute Treatment and Prevention of Menstrually Related Migraine Headache: Evidence-Based Review". Neurology 70 (17): 1555–1563. doi:10.1212/01.wnl.0000310638.54698.36. PMID 18427072.
- National formulary of India, 4th Ed. New Delhi, India, Indian Pharmacopoeia commission; 2011: 7
- Drugs for Osteoarthritis. The Medical Letter, 56(1450):80-84, September 2014
- https://www.medicinescomplete.com/mc/bnf/current/PHP6487-mefenamic-acid-non-proprietary.htm accessed 19th sept 2014
- "Side effects for Mefenamic Acid". Medline Plus. National Institutes of Health.
- Trinus, F. P.; Mokhort, N. A.; Yagupol'skii, L. M.; Fadeicheva, A. G.; Danilenko, V. S.; Ryabukha, T. K.; Fialkov, Yu. A.; Kirichek, L. M.; Endel'man, É. S.; Get'man, G. A. (1977). "Mefenamic acid — A Nonsteroid Antiinflammatory Agent". Pharmaceutical Chemistry Journal 11 (12): 1706–1711. doi:10.1007/BF00778304.
- MedlinePlus Drug Information: Mefenamic Acid. Last accessed September 28, 2005.
- Ponstel Pharmacology, Pharmacokinetics, Studies, Metabolism - Mefenamic Acid - RxList Monographs. Last accessed September 28, 2005.
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