Magnesium: side effects, risks, and what to do
Magnesium is generally well tolerated. The most common adverse effects are digestive (laxative effect) and depend mainly on dose and salt form. More rarely—especially with kidney impairment—true hypermagnesemia can occur and requires medical attention. We also answer: does magnesium in the evening disrupt sleep?
The most common side effects
They are usually mild and reversible by stopping or lowering the dose:
- Diarrhea or loose stools, sometimes abdominal cramps and bloating
- Nausea, occasional vomiting or a metallic taste
- Fatigue or heaviness if your personal dose is too high
These effects mostly reflect the osmotic effect of certain salts (citrate, oxide, chloride). Health agencies note that the excess from supplements is the main driver of these digestive issues according to the NIH ODS magnesium factsheet.
Red flags: when to worry
True hypermagnesemia is uncommon and primarily occurs with renal insufficiency or very high intakes (IV use, long‑term magnesium laxatives). Possible signs:
- Unusual drowsiness, confusion, reduced reflexes
- Low blood pressure, bradycardia, palpitations, faintness
- Marked muscle weakness, breathing difficulty
If these occur, seek urgent care. Diagnosis relies on serum magnesium testing and clinical assessment; see hypermagnesemia and manifestations.
Altered consciousness, respiratory distress, marked bradycardia, severe weakness: call emergency services.
Tolerance by form and dose
Dose and salt form drive tolerance:
- Salts with a higher osmotic effect (often citrate, oxide, chloride) more easily cause a laxative effect at high dose
- Many people tolerate forms like bisglycinate better (at equivalent elemental dose)
- Splitting the dose (AM/PM) and taking it with a meal improves GI tolerance
For fundamentals (intake, sources, choosing forms), see our magnesium guide and citrate / bisglycinate pages.
Start low, increase gradually, split doses, take with meals, and hydrate. If discomfort appears, lower the dose or trial a different form.
Drug interactions and precautions
Magnesium can reduce the absorption of some medicines; separate by 2–4 hours. Common cases: quinolone and tetracycline antibiotics, and thyroid hormones (levothyroxine). See the NIH ODS interactions summary.
If you have kidney disease, take multiple medicines (cardio drugs, diuretics, long‑term PPIs), or persistent GI disorders, seek medical advice before supplementing.
Magnesium at night: does it disrupt sleep?
Generally no: magnesium is not a stimulant and is often associated with relaxation (nervous system, muscles). Correcting deficiency may even improve sleep for many people.
However:
- A late dose can cause GI discomfort (or faster transit) that might wake you
- Combination formulas (other actives) or large single doses may be less tolerated late in the evening
In practice: try dosing 2–3 hours before bedtime, with dinner, and split if needed. For form/tolerance comparisons, see magnesium citrate.
How much without discomfort?
Recommended intakes vary by source (≈300–380 mg/day in adults). To limit supplement‑related diarrhea, some authorities define a tolerable upper limit from supplements only:
- EFSA: about 250 mg/day of elemental magnesium from supplements for adults (GI tolerance)
- NIH: 350 mg/day from supplements/medicines
These do not apply to dietary magnesium. The goal isn’t to “max out,” but to find the lowest effective, well‑tolerated dose, preferably split.
What to do if side effects appear
- Stop or reduce temporarily and hydrate
- Restart lower, split (AM/PM), and take with meals
- Trial a different form (e.g., bisglycinate)
- If you have kidney disease or sensitive medications, seek medical advice
- If red flags occur, seek urgent care
Quick FAQ
Can magnesium at night keep me awake?
Generally no; it supports relaxation. Avoid large doses right before bed if you have a sensitive gut.
Why do I get diarrhea with magnesium?
Dose‑dependent osmotic effect. Lower the dose, split, take with meals, or switch forms.
Which medicines should I not combine with magnesium?
Quinolone/tetracycline antibiotics and levothyroxine: separate by 2–4 h; see documented interactions.
Who should get medical advice first?
Renal disease, chronic conditions, polypharmacy, pregnancy/breastfeeding.
