Mefenamic acid
NSAIDs
Nonsteroidal anti-inflammatory drugs (NSAIDs)Pregnancy: avoid
Avoid from week 20: from about 20 weeks NSAIDs can reduce amniotic fluid through their effect on the fetal kidney, and from about 30 weeks they can close the fetal ductus…
A fenamate NSAID for short-term pain and period pain (dysmenorrhoea). Use for pain is limited to one week; it carries all NSAID heart and gut warnings.
Quick facts
- Group
- Nonsteroidal anti-inflammatory drugs (NSAIDs)
- Target
- Cyclooxygenase 1 and 2 (COX-1/COX-2)
- Used for
- Primary dysmenorrhoea
- Key effects
- Abdominal pain · Diarrhoea · Headache and dizziness
- Monitor
- Blood pressure · Signs of gastrointestinal bleeding
- ATC
- M01AG01
No doses: the clinician sets them by the current guideline.
Mechanism of action
The figure shows mefenamic acid itself in the active site of human COX-2.
Mefenamic acid inhibits COX-1 and COX-2 and so lowers prostaglandin production. In the uterus, less prostaglandin means weaker, less painful contractions, which is why it is used for primary dysmenorrhoea.
- Blocks COX-1 and COX-2
- Less prostaglandin is made in tissue and uterus
- Less pain and weaker uterine cramps
Indications
- Primary dysmenorrhoea Dysmenorrhoea →
- Short-term mild to moderate pain (from 14 years, no more than one week)
Adverse effects
Common
- Abdominal pain, dyspepsia and heartburn
- Diarrhoea, nausea and vomiting
- Headache and dizziness
- Rash and tinnitus
Contraindications
- Asthma, hives or allergic-type reactions after aspirin or another NSAID
- Pain around coronary artery bypass graft (CABG) surgery
- Hypersensitivity to mefenamic acid
Key interactions
- Warfarin, antiplatelets, SSRIs and SNRIs
- More bleeding, mainly in the gut
- ACE inhibitors, ARBs and diuretics
- Weaker blood-pressure and diuretic effect; risk of kidney injury
- Lithium and methotrexate
- Higher levels and toxicity of lithium or methotrexate
What is monitored
- Blood pressure, swelling and signs of heart failure
- Signs of gastrointestinal bleeding (black stools, vomiting blood, anaemia)
- Kidney function, potassium, liver tests and blood count in long-term use
Administration and safety
- Given by mouth at the lowest effective dose for the shortest time
- For pain, treatment lasts no more than one week
Pregnancy and breastfeeding
Avoid from week 20: from about 20 weeks NSAIDs can reduce amniotic fluid through their effect on the fetal kidney, and from about 30 weeks they can close the fetal ductus arteriosus too early.
Nursing care
- Ask about ulcers, bleeding, kidney or heart disease and aspirin-sensitive asthma before the first dose
- Check blood pressure and fluid status during treatment
- Teach to report black stools, vomiting blood, chest pain, weakness on one side or rash
- Teach to avoid taking another NSAID at the same time, including over-the-counter ones
Pharmacokinetics
- Metabolism
- By CYP2C9 to hydroxylated products, then glucuronidation
- Elimination
- About half in the urine, the rest largely in the faeces
- Half-life
- Short, about 2 hours
Test yourself
Think of the answer, then tap the question to see it.
For which gynaecological pain is mefenamic acid used?
Primary dysmenorrhoea.
What limit applies to its use for general pain?
No more than one week.
Which CYP enzyme metabolises mefenamic acid?
CYP2C9.
Connections
← Back to the class: Nonsteroidal anti-inflammatory drugs (NSAIDs)
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - MEFENAMIC ACID capsule (DailyMed (U.S. National Library of Medicine), 2026); 5IKR: The Structure of Mefenamic Acid Bound to Human Cyclooxygenase-2 (RCSB Protein Data Bank, 2016); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).
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Representative model, for educational purposes only.