So you’ve started taking magnesium for better sleep, stronger bones, or muscle recovery. Now you’re running to the bathroom. You’re not alone. Diarrhoea and loose stools are the most common complaint about magnesium supplements. The good news? It’s not magnesium itself that’s the problem. It’s the form you’re taking.
Yes, some forms of magnesium cause diarrhoea, but others (particularly magnesium glycinate) don’t. Switch forms and you can keep the benefits without the side effect.
Why Some Forms of Magnesium Cause Loose Stools
Most magnesium supplements that cause diarrhoea work the same way: they pull water into your large intestine. This is called an osmotic laxative effect, and it’s actually the reason some people use magnesium supplements specifically for constipation relief.
When you take magnesium oxide, magnesium citrate, or magnesium carbonate, your body struggles to absorb all of it in the small intestine. The excess magnesium that isn’t absorbed continues into your colon, where it draws water along with it, leading to softer, looser stools. At higher doses, this becomes obvious diarrhoea.
The cheaper magnesium tablets and supplements you’ll find in supermarkets are usually oxide or carbonate forms. Magnesium oxide, for example, has a bioavailability of roughly 4% — meaning most of what you swallow never reaches your cells. It reaches your colon instead.
A 2023 perspective article reviewed ten randomised controlled trials and found that seven of them reported no significant difference in diarrhoea between magnesium and placebo groups, even at doses up to 1,200 mg/day (Mah & Bhaskaran, 2023). The consistent finding: the form of magnesium matters more than the dose.
Which Forms of Magnesium Are Most Likely to Cause It
Not all magnesium supplements are equal. Here’s a breakdown of the most common forms and their laxative risk:
| Magnesium Form | Laxative Risk | Why? |
|---|---|---|
| Magnesium oxide | Very high | Very poorly absorbed (~4% bioavailability); strong osmotic effect |
| Magnesium citrate | Medium–high (dose-dependent) | Better absorbed than oxide, but still osmotic at higher doses |
| Magnesium sulfate (Epsom salt) | High | Variable absorption; used medically as a laxative |
| Magnesium hydroxide (Milk of Magnesia) | High | Moderate absorption; sold specifically as a laxative/antacid |
| Magnesium carbonate | Medium | Moderate absorption; some osmotic effect |
| Magnesium glycinate | Low | Absorbed via amino acid transporters; no osmotic pull |
| Magnesium malate | Low | Well-absorbed; minimal osmotic effect |
If it says magnesium oxide, sulfate, or high-dose citrate, that’s your culprit.
Why Magnesium Glycinate Is Different
Chelated magnesium (specifically magnesium glycinate, also called magnesium bisglycinate) works in a fundamentally different way. Instead of relying on osmotic pull, it’s absorbed directly through your small intestine via amino acid transporters, which recognise the glycine chelation complex.
Because magnesium glycinate is absorbed early and efficiently in your small intestine, it never reaches your colon in high enough concentrations to trigger water retention. No osmotic effect, no loose stools. Even at therapeutic doses.
Research comparing different forms of magnesium consistently shows that glycinate has superior bioavailability and produces fewer gastrointestinal side effects than oxide or standard citrate forms. A 2005 study by Coudray et al. in Magnesium Research found chelated organic forms significantly outperformed inorganic forms on absorption measures.
Other Reasons Magnesium Might Upset Your Stomach
Even with a well-absorbed form, there are a few other factors that can cause digestive discomfort:
- Dose too high too soon. Start with a lower dose (100–200mg elemental magnesium daily) and build up. Your digestive system needs time to adjust.
- Taking the full dose in one go. Splitting your intake (for example, 200mg in the morning and 200mg in the evening) is easier on your gut than taking 400mg at once.
- Taking it on an empty stomach. Taking any supplement without food can cause nausea or mild stomach upset. Glycinate is more forgiving than most forms, but a light snack alongside it removes the risk entirely.
- Medication interactions. Certain antibiotics, blood pressure medications, and diuretics can interact with magnesium. Check with your pharmacist if you’re on multiple medications.
- Underlying digestive sensitivity. If you have IBS or inflammatory bowel disease, glycinate is the safest starting point, but it’s still worth consulting your GP before supplementing.
When to Stop and See Your GP
Mild loose stools that settle within a day or two of adjusting your dose aren’t a concern. But contact your GP if you experience severe or persistent diarrhoea lasting more than two to three days, signs of dehydration (dizziness, dark urine, excessive thirst), or unusual symptoms like muscle weakness or heart palpitations, which can signal an electrolyte imbalance.
FAQ
Q: Can magnesium cause diarrhoea?
A: Yes, certain forms can. Magnesium oxide, carbonate, and high-dose citrate are poorly absorbed, so unabsorbed magnesium pulls water into the colon and causes loose stools or diarrhoea. Well-absorbed forms like magnesium glycinate work differently and rarely cause this problem.
Q: What is the maximum magnesium dose before it causes a laxative effect?
A: It depends on the form. With magnesium oxide or citrate, laxative effects can start at 200–400mg. With magnesium glycinate, most people tolerate 300–400mg without issues. The EFSA upper safe limit for supplemental magnesium is 400mg/day. A 2023 review of ten RCTs found that seven reported no significant diarrhoea vs placebo even at higher doses, provided a well-absorbed form was used (Mah & Bhaskaran, 2023). If loose stools start, try splitting your dose or reducing it before switching forms.
Q: How do I stop loose stools from magnesium supplements?
A: Check the form first. Oxide and high-dose citrate are the most common culprits. Switch to magnesium glycinate, reduce your dose, split your daily intake across two servings, and always take it with food. Most people see improvement within 2–3 days.
Q: Does magnesium glycinate cause diarrhoea or loose stools?
A: Rarely. Unlike magnesium oxide or high-dose magnesium citrate, glycinate is absorbed via amino acid transporters in the small intestine rather than triggering an osmotic effect in the colon. It’s the form most often recommended for people who’ve had digestive problems with other magnesium supplements.
Q: Which magnesium is least likely to cause loose stools?
A: Magnesium glycinate and magnesium malate both have low laxative risk. Glycinate is generally the first choice because it also has calming properties and is well-suited to daily use. Avoid oxide and high-dose citrate if digestive sensitivity is a concern.
Q: How long until digestive issues stop after switching to glycinate?
A: Most people notice improvement within 2–3 days of switching. If issues continue beyond a week, the problem may be dose-related rather than form-related. Try reducing and building back up gradually.
Q: Is magnesium oxide ever appropriate?
A: Magnesium oxide is useful for short-term constipation relief; the osmotic laxative effect is intentional in that context. It’s not a good choice as a daily supplement for general health, where bioavailability and tolerability matter.
Q: What dose of magnesium glycinate should I take?
A: A typical daily dose for general health support is 200–300mg elemental magnesium, taken with food. The UK NHS recommends 300mg/day for men and 270mg/day for women from all sources combined. The EFSA upper safe limit for supplemental magnesium is 400mg/day.
Q: What should I look for when buying magnesium for a sensitive stomach in the UK?
A: Look for a supplement that uses magnesium glycinate (bisglycinate) as the sole magnesium source — not a blend that includes oxide or carbonate. Check the label for elemental magnesium content, not just total capsule weight. Avoid products with unnecessary fillers, bulking agents, or added sugars. A UK-based manufacturer with third-party testing is a good sign of quality control.
Q: Do magnesium supplements with added probiotics or digestive enzymes reduce diarrhoea risk?
A: Some brands add probiotics or digestive enzymes to magnesium supplements, but the evidence that these additions reduce laxative side effects is limited. The form of magnesium itself is the primary factor. Switching from oxide to glycinate will make a far bigger difference than any added ingredient. Keep it simple: choose the right form, take it with food, and start at a lower dose.
The Bottom Line
Magnesium itself doesn’t cause diarrhoea — cheap, poorly absorbed forms do. Switching to magnesium glycinate gives you better absorption, far fewer digestive side effects, and the same (or better) health benefits.
If you’ve been put off magnesium because of previous digestive complaints, it’s worth trying again with the right form. Our Magnesium Glycinate is a pure bisglycinate chelate with no oxide blending and no unnecessary fillers.
References
- Coudray C, et al. (2005). Study of magnesium bioavailability from ten organic and inorganic Mg salts. Magnesium Research, 18(4), 215–223.
- Mah J, Bhaskaran N. (2023). Perspective: Call for re-evaluation of the tolerable upper intake level for magnesium supplementation in adults. PMC10509448
- Mori H, et al. (2021). The effect of magnesium oxide ingestion on the human colon. Nutrients, 13(2), 607.
- Walker AF, et al. (2003). Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnesium Research, 16(3), 183–191.
- NHS. Vitamins and minerals — Magnesium.
This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before starting any supplement, especially if you are pregnant, have a medical condition, or take prescription medication.
