Mefenamic Acid
Is It Right for You?
A complete guide to Mefenamic Acid — what it treats, how it works, dosages, side effects, and when a clinician may prescribe it following an online consultation.
What Is Mefenamic Acid?
Mefenamic acid is a non-selective non-steroidal anti-inflammatory drug (NSAID) of the fenamate class, available in capsules of 250 mg and tablets of 500 mg. It is used primarily for the short-term management of acute pain and, distinctively within the NSAID class, has a specific and well-established role in the treatment of primary dysmenorrhoea (painful periods) and heavy menstrual bleeding.
Unlike most NSAIDs, which are prescribed principally for musculoskeletal and inflammatory pain, mefenamic acid is most commonly encountered in gynaecological clinical practice for menstrual disorders. It reduces both the severity of period pain and the volume of menstrual blood loss — a dual benefit that makes it particularly valuable as a non-hormonal first-line option for women with heavy and/or painful periods who do not wish to use or cannot tolerate hormonal contraceptives or other hormonal therapies.
Mefenamic acid is a prescription-only medication in the UK. A clinician must assess the nature and cause of pain or menstrual disturbance before prescribing.
What Conditions Is Mefenamic Acid Used For?
Mefenamic acid is indicated for:
first-line non-hormonal pharmacological treatment for painful menstruation; most effective when started 1–2 days before anticipated period onset and taken regularly throughout the heaviest, most painful days
Reduces menstrual blood loss by 25–35% through inhibition of prostaglandin-driven endometrial vasodilatation; used alongside or instead of tranexamic acid
short-term management of mild-to-moderate acute pain, including headache, dental pain, and post-operative pain; for up to 7 days
In dysmenorrhoea, mefenamic acid treats the pain of menstruation but does not address underlying causes of secondary dysmenorrhoea (such as endometriosis, fibroids, or adenomyosis). If period pain is severe, worsening, or associated with other symptoms (deep dyspareunia, subfertility, or cyclical bowel symptoms), gynaecological assessment to exclude endometriosis and other pathology is essential.
How Does Mefenamic Acid Work?
Mefenamic acid's anti-inflammatory, analgesic, and antipyretic effects, as well as its specific benefits in dysmenorrhoea and menorrhagia, arise from its non-selective inhibition of the cyclooxygenase (COX) enzyme system:
Non-selective COX-1 and COX-2 inhibition: Mefenamic acid competitively and reversibly inhibits both COX-1 (constitutively expressed) and COX-2 (inducible at sites of inflammation) isoforms. This prevents the conversion of arachidonic acid to prostaglandin endoperoxides (PGG2 and PGH2) — the precursors to the prostaglandins, thromboxane, and prostacyclin that mediate inflammation, pain, fever, platelet aggregation, and vascular tone.
Role in dysmenorrhoea: Primary dysmenorrhoea is driven by excessive production of prostaglandin F2α (PGF2α) and prostaglandin E2 (PGE2) by the shedding endometrium during menstruation. These prostaglandins cause intense uterine smooth muscle contractions, myometrial ischaemia, and sensitisation of pelvic pain receptors — producing the cramping pelvic pain of primary dysmenorrhoea. By inhibiting COX enzymes, mefenamic acid directly reduces prostaglandin synthesis within the uterus, attenuating the uterine contractions and pain.
Role in menorrhagia: Excess prostaglandins also promote endometrial vasodilation and reduce platelet aggregation in the endometrial vasculature, contributing to heavy menstrual blood loss. COX inhibition reduces these prostaglandin-mediated effects, decreasing menstrual flow volume.
Dosages & Administration
The correct dose depends on the type and severity of infection, age, weight, and kidney function. Always follow your clinician's instructions. The table below is for general reference only.
Administration Tips
Always take with food or milk. Mefenamic acid has a higher propensity for gastrointestinal irritation than some other NSAIDs; taking it with food or milk substantially reduces gastric discomfort, nausea, and the risk of GI mucosal injury.
Start before pain begins for dysmenorrhoea. Beginning mefenamic acid 1–2 days before the anticipated start of menstruation — before prostaglandin levels in the uterus have peaked — provides superior pain control compared to starting only after pain is established.
Do not exceed 7 days for acute pain or 5 days for menstrual indications. Mefenamic acid is not suitable for chronic pain management.
Do not take alongside other NSAIDs. Concurrent use of two NSAIDs provides no additional analgesic benefit and significantly increases gastrointestinal and renal risk.
Stay well hydrated. Adequate fluid intake reduces the risk of NSAID-associated renal effects.
Side Effects of Mefenamic Acid
Most side effects are mild and resolve on their own. Serious side effects are rare but require immediate medical attention.
- Nausea, dyspepsia, and abdominal discomfort --- the most common; significantly reduced by taking with food
- Diarrhoea --- particularly common with mefenamic acid compared to other NSAIDs; dose-related
- Headache
- Dizziness
- Oedema (fluid retention)
- Elevated liver enzymes (transient and usually mild)
- Gastrointestinal ulceration, bleeding, or perforation --- as with all non-selective NSAIDs; presents as black or tarry stools, vomiting blood, or severe abdominal pain; risk is higher with prolonged use, in elderly patients, and in those with a prior GI history
- Haemolytic anaemia --- mefenamic acid is more commonly associated with haemolytic anaemia than other NSAIDs; report any new pallor, jaundice, or dark urine
- Seizures --- mefenamic acid overdose (and rarely at therapeutic doses) is associated with tonic-clonic seizures; this is a class-distinguishing adverse effect of the fenamate group; any seizure requires emergency assessment
- Acute kidney injury --- COX inhibition reduces renal prostaglandin synthesis; risk is higher in volume-depleted patients, the elderly, and those with pre-existing renal disease
Mefenamic acid, uniquely among commonly prescribed NSAIDs, is associated with seizures at overdose and rarely at high therapeutic doses. This is an inherent property of the fenamate class and is not shared by ibuprofen, naproxen, or diclofenac. Patients with a history of seizures or epilepsy should use mefenamic acid with additional caution.
Drug Interactions
Always disclose all prescription drugs, over-the-counter medicines, vitamins, and supplements to your clinician or pharmacist before starting Mefenamic Acid
NSAIDs reduce renal lithium clearance, raising plasma lithium levels and risk of toxicity; monitor lithium levels closely.
NSAIDs reduce methotrexate renal clearance; risk of methotrexate toxicity; avoid.
additive nephrotoxicity.
may reduce mefenamic acid absorption; separate doses by at least 2 hours.
Important Warnings
Mefenamic acid, like all non-selective NSAIDs, carries a risk of serious GI events including peptic ulceration, haemorrhage, and perforation — which can be fatal and may occur without prior warning symptoms. Co-prescribing a proton pump inhibitor (e.g., omeprazole) is recommended in patients with GI risk factors including older age, prior peptic ulcer, and concurrent aspirin or anticoagulant use.
All NSAIDs increase the risk of thrombotic cardiovascular events — myocardial infarction and stroke. This risk increases with dose and duration. Mefenamic acid must be used at the lowest effective dose for the shortest time and is contraindicated in patients with established ischaemic heart disease, peripheral arterial disease, or stroke.
Patients with asthma, chronic rhinosinusitis with nasal polyps, and aspirin sensitivity (Samter's triad) are at risk of severe bronchospasm and anaphylaxis with any non-selective NSAID. Mefenamic acid is contraindicated in patients with known NSAID-induced bronchospasm or aspirin hypersensitivity.
Mefenamic acid is contraindicated from 20 weeks of gestation onwards due to the risk of premature closure of the fetal ductus arteriosus and fetal renal impairment with oligohydramnios. It must not be used in the third trimester.
Speak to a Clinician About Treatment
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The treatment you need, when you need it.
Mefenamic Acid FAQs
All non-selective NSAIDs reduce prostaglandin synthesis and can relieve dysmenorrhoea, but mefenamic acid has the dual advantage of reducing both period pain and menstrual blood loss in the same treatment. Its long-standing use for dysmenorrhoea means it has a well-characterised evidence base specifically in this indication, and it remains a first-line NICE-recommended option for primary dysmenorrhoea and heavy menstrual bleeding.
For best results, start taking mefenamic acid 1–2 days before your period is due to begin, before prostaglandin levels have peaked in the uterus. Starting after pain is already established provides less effective control. Continue taking it regularly (every 8 hours with food) for the duration of your heaviest and most painful days — typically 3–5 days.
Yes — mefenamic acid can be used cyclically each month for menstrual indications, using it only during the days of your period. However, it should not be taken continuously outside of menstrual episodes without clinical review, as it is intended for short-term use only.
Like other NSAIDs, mefenamic acid may theoretically impair ovulation through inhibition of prostaglandin-mediated follicular rupture (luteinised unruptured follicle syndrome) with prolonged mid-cycle use. Women trying to conceive should avoid NSAIDs around the time of ovulation and discontinue use if pregnancy is confirmed.
If mefenamic acid at the recommended dose does not provide sufficient relief, discuss with your clinician. Options include combining mefenamic acid with tranexamic acid for heavy menstrual bleeding, considering hormonal treatments (combined oral contraceptive pill, progestogen-releasing IUS/Mirena), or arranging gynaecological assessment to exclude secondary causes such as endometriosis.