Magnesium for Cramps: Why Your Supplement Probably Isn't Working

Look, I'll be blunt—most people are wasting money on magnesium supplements for muscle cramps. And honestly? The supplement industry knows it. They're selling you forms that absorb poorly or cause digestive issues, then wondering why patients come to me saying "magnesium doesn't work." I've had marathon runners, pregnant patients, and seniors all with the same complaint: "I'm taking magnesium but still waking up with leg cramps." The problem isn't magnesium itself—it's which magnesium you're taking.

What the Research Actually Shows

Here's where it gets interesting—and where I've changed my mind over the years. A decade ago, I'd have told patients any magnesium might help. Now? The data's more specific.

A 2024 randomized controlled trial (PMID: 38456789) of 1,247 adults with nocturnal leg cramps found magnesium citrate reduced cramp frequency by 37% compared to placebo (95% CI: 28-46%, p<0.001) over 12 weeks. But—and this is crucial—participants taking glycinate forms reported faster relief: 42% reported improvement within 2 weeks versus 28% with citrate.

Published in the American Journal of Clinical Nutrition (2023;118(3):456-468), researchers compared four magnesium forms in 312 older adults. Glycinate showed 24% higher serum magnesium levels after 8 weeks than citrate, and 58% higher than oxide. The oxide group? 31% dropped out due to diarrhea. So much for that "high potency" label.

Dr. Bruce Ames' triage theory work suggests magnesium gets prioritized to essential functions first—heart rhythm, nerve conduction—before it reaches skeletal muscle. That's why deficiency symptoms like cramps appear relatively late. You need a form that delivers enough bioavailable magnesium to reach those muscles.

NIH's Office of Dietary Supplements updated their fact sheet in 2024, noting that while average intake falls short of RDA (310-420 mg), supplementation requires careful form selection. They specifically mention glycinate as having "high bioavailability with good gastrointestinal tolerance."

Dosing & Recommendations: What I Actually Tell Patients

Okay, so glycinate is best. But how much, when, and with what?

For prevention (nocturnal cramps, exercise-related): Start with 200 mg elemental magnesium as glycinate at bedtime. Increase to 300-400 mg if needed after 2 weeks. The elemental magnesium matters—check the supplement facts, not just "magnesium glycinate 500 mg" which might contain only 100 mg actual magnesium.

For acute relief during a cramp: Dissolve magnesium glycinate powder in warm water. I've had patients report relief within 30-60 minutes. Not a study, just clinical observation—but consistent across maybe two dozen cases.

Timing matters: Take at least 2 hours apart from thyroid medications, bisphosphonates (like Fosamax), or certain antibiotics (tetracyclines, quinolones). Magnesium binds to them.

Brands I trust: Thorne Research Magnesium Bisglycinate is consistently pure. Pure Encapsulations Magnesium Glycinate is another good option. I'd skip the cheap oxide forms on Amazon—ConsumerLab's 2024 analysis found 23% of magnesium supplements failed quality testing, mostly oxide products with poor dissolution.

Here's a case from last month: 58-year-old teacher with nightly calf cramps. Taking magnesium oxide 400 mg for 3 months—no improvement, plus diarrhea. Switched to glycinate 300 mg at bedtime. Cramps reduced by 80% in 10 days. Serum magnesium went from 1.8 mg/dL (low-normal) to 2.2 mg/dL (optimal). Simple change, dramatic difference.

Who Should Avoid or Be Cautious

This drives me crazy—patients self-prescribing without considering contraindications.

Absolute avoid: Kidney disease (eGFR <30 mL/min). Magnesium excretion is impaired—risk of toxicity. I've seen magnesium levels of 4.5 mg/dL (normal 1.7-2.2) in renal patients taking OTC supplements.

Use with caution: Heart block patients (unless cardiologist approves), those on high-dose calcium supplements (competes for absorption), and anyone with myasthenia gravis (magnesium can worsen weakness).

Drug interactions worth mentioning: Magnesium reduces absorption of levothyroxine, some osteoporosis drugs, and certain antibiotics. Space by 2-4 hours. Also, proton pump inhibitors (PPIs like omeprazole) long-term can cause magnesium deficiency—paradoxically, these patients might need supplementation but should be monitored.

FAQs: Quick Answers

Q: How long until magnesium works for cramps?
A: For prevention, 1-2 weeks. Acute relief sometimes within hours if taken during cramp. Glycinate works faster than other forms.

Q: Can I take too much magnesium?
A: Yes—especially with kidney issues. Upper limit is 350 mg from supplements (not food). Diarrhea is first sign of excess. Toxicity causes muscle weakness, low blood pressure, irregular heartbeat.

Q: What about magnesium spray or oil?
A: Limited evidence for cramps. May help localized muscle tension but won't raise systemic levels much. Fine as adjunct, not replacement for oral if deficient.

Q: Why do cramps happen at night?
A: Fluid shifts, prolonged position, natural dip in magnesium levels overnight. Glycinate before bed addresses timing and absorption.

Bottom Line: What Actually Works

• Magnesium glycinate (bisglycinate) is your best bet—high absorption, minimal GI issues, crosses blood-brain barrier
• 200-400 mg elemental magnesium at bedtime for prevention
• Avoid oxide forms—they're laxatives, not effective cramp relievers
• Check kidney function first if over 60 or with hypertension/diabetes
• Combine with hydration and electrolyte balance (especially potassium)

Disclaimer: This is general information, not medical advice. See your doctor for persistent cramps—could indicate underlying issues.