The short answer
Magnesium hydroxide can increase mefenamic acid blood levels. The label advises avoiding concomitant antacids; ask your pharmacist about an alternative instead of relying on untested spacing.
What this answer covers
Oral magnesium hydroxide antacid immediately following oral mefenamic acid, bridged to exact mefenamate IN. Other magnesium salts, foods and routes are not equivalent.
- Supplement or preparation
- Magnesium hydroxide
- Medication ingredient
- mefenamate
A small human study measured higher exposure
In six healthy volunteers, magnesium hydroxide given immediately after mefenamic acid accelerated absorption. The current label reports that 1.7 g with 500 mg mefenamic acid increased peak concentration by 125% and total exposure by 36%. These are drug-level measurements, not a measured rate of side effects.
The exact magnesium ingredient matters
This evidence concerns magnesium hydroxide used as an antacid. It does not establish the same effect for every magnesium supplement, food source or salt. Bring the ingredient and amount to the pharmacist, including combination antacid or laxative products.
Ask about an alternative rather than guessing a gap
The mefenamic acid label advises avoiding concomitant antacids because increased absorption could increase adverse reactions. Neither the small study nor the label establishes a number of hours that prevents the interaction. Ask the pharmacist or prescriber how to manage the antacid need.
Do not change the prescription dose yourself
Higher measured exposure is a reason to review co-use, not to calculate your own dose reduction. Follow the prescription’s short treatment limits and seek urgent care for vomiting blood or black, tarry stools. Persistent stomach symptoms deserve assessment rather than repeated self-treatment.
Evidence and practical considerations
Absorption & effectiveness
Magnesium hydroxide can increase mefenamic acid absorption and peak blood levels.
- Exact scope
- Oral magnesium hydroxide antacid immediately following oral mefenamic acid, bridged to exact mefenamate IN. Other magnesium salts, foods and routes are not equivalent. with Exact mefenamate IN 257844 linked by current RxNorm to mefenamic acid PIN 6693. The inspected oral acid capsules supply the explicit ingredient bridge, not meclofenamate or a different fenamate.
- Medication form and route
- Oral
What remains uncertain
- Small healthy-volunteer study; dose and timing specific. No measured clinical toxicity increase or validated spacing interval. Other magnesium forms were not established.
Factors that may matter: Exact preparation and dose; Fasting versus fed dosing; Bleeding and ulcer history; Other medicines.
Read the supporting source summaries
Source 1: Current prescribing information
dailymed.nlm.nih.gov · Source check Sep 10, 2026
Population: Prescription oral oral mefenamic acid 250 mg capsules users; short human pharmacology studies where described.
Short-term acute pain up to seven days and primary dysmenorrhea indications. Serious GI warnings; magnesium hydroxide raises peak concentration and AUC in a small human study.
How this applies: Exact current product label. Exact mefenamate IN 257844 linked by current RxNorm to mefenamic acid PIN 6693. The inspected oral acid capsules supply the explicit ingredient bridge, not meclofenamate or a different fenamate.
Exact oral product label; no measured botanical co-use incidence. Ingredient, route, preparation and comparator differences must be retained.
Source 2: Original clinical research
pubmed.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: Six healthy volunteers in the mefenamic-acid crossover component.
Magnesium hydroxide accelerated absorption and increased peak levels; exact current label reports 125% higher peak and 36% higher total exposure with 1700 mg.
How this applies: Direct oral mefenamic acid and magnesium hydroxide study, with explicit RxNorm mefenamate-to-mefenamic-acid bridge.
Abstract truncated at 250 words. Tolfenamic-acid study components are separate; its sodium bicarbonate results do not apply to mefenamic acid or tolmetin. No measured adverse-event increase or fixed spacing solution.
Source 3: Medication terminology
rxnav.nlm.nih.gov · Source check Sep 10, 2026
Population: Terminology.
mefenamate is IN. Exact mefenamate IN 257844 linked by current RxNorm to mefenamic acid PIN 6693. The inspected oral acid capsules supply the explicit ingredient bridge, not meclofenamate or a different fenamate.
How this applies: Exact identity.
Not clinical evidence.
Research assessment: · Open full citations ↗
Review and change history
- Publication status
- Published research summary
- Clinical review
- Not yet recorded
| Date | Update |
|---|---|
| Article prepared | |
| Sources checked and article drafted. | |
| Research summary published |
Article revision: a4f2d22f32136712
This guide brings together the evidence records used by our interaction checker. Each finding retains its own preparation limits, research date, and source summaries.
Full citations require verification. Reading this answer and the source summaries does not.
Taking other supplements or medications?
Use the checker to explore your other combinations. Ask a pharmacist to review the exact products, amounts and medical conditions in your complete list.
Open the interaction checkerInformation, not medical advice. Do not change prescribed treatment based on this guide.