Magnesium
Serum Magnesium
What this test measures
Magnesium is a mineral essential for muscle and nerve function, blood sugar control, and bone health. Your body needs it for over 300 different processes.
Understanding the numbers
Typical range: 0.7-1.0 mmol/L
Magnesium levels can be affected by certain medications, alcohol, and digestive conditions.
- Low: 0.3–0.7 mmol/L
- Normal: 0.7–1.0 mmol/L
- Raised: 1.0–2.0 mmol/L
Key numbers
- 0.7 mmol/L — Lower limit — below = symptoms possible
- 0.5 mmol/L — Often symptomatic — replace
Typical UK range. Low magnesium is common with diuretics, PPIs, alcohol, and malabsorption (gut, coeliac, Crohn's). Low magnesium often coexists with low potassium and must be corrected to allow potassium replacement to work. High magnesium is almost always from kidney failure plus magnesium-containing antacids or laxatives.
Ranges can vary slightly between laboratories — your report may show slightly different numbers. This page does not read or interpret your result; your GP looks at it alongside your history and other tests.
What a higher-than-normal result can mean
- Usually caused by kidney problems (kidneys can't remove excess magnesium)
- Can be caused by taking too many magnesium supplements or antacids
- May cause muscle weakness, low blood pressure, or nausea
- Rarely, can affect your heart rhythm
Questions you could ask your GP
- What might be causing my abnormal magnesium level?
- Do I need to take magnesium supplements?
- Could any of my medications be affecting my magnesium?
- Should I make any changes to my diet?
Example stories
These are made-up examples to show how results are talked about in real appointments. They are not real patients.
PPI-induced low magnesium
Ronald had been on omeprazole for years. He had muscle cramps at night. Magnesium 0.48, potassium 3.2.
What the GP said: “Long-term PPI use can deplete magnesium, and low magnesium often drags potassium down too. We'll replace both and discuss whether you still need the PPI at your dose.”
What happened next: With magnesium supplements and reducing the PPI, magnesium rose to 0.8 and his cramps resolved.
PPIs — especially long-term — are a recognised cause of low magnesium. Review PPI use when magnesium is low; you may be able to lower the dose or switch to an H2 blocker.
Alcohol-related low magnesium
Vipin drank heavily. He had tremor and palpitations. Magnesium 0.55, potassium 3.0, raised GGT, and was admitted for alcohol withdrawal.
What the GP said: “Heavy drinking lowers magnesium and potassium and raises the risk of serious withdrawal including fits. We'll replace both carefully, give you Pabrinex, and follow withdrawal protocols.”
What happened next: After inpatient detox with replacement, Vipin's magnesium and potassium normalised.
Alcohol-dependent patients often have low magnesium. Correcting it reduces arrhythmia risk and helps potassium replacement work.
Malabsorption from Crohn's
Sarah had active Crohn's disease with significant diarrhoea. Magnesium 0.44, often struggled to keep it up with oral tablets.
What the GP said: “Your gut isn't absorbing magnesium well. Oral isn't enough here — we'll arrange periodic IV magnesium until your Crohn's is better controlled.”
What happened next: With better Crohn's control and IV magnesium every few weeks, her magnesium stayed around 0.7.
Gut disease can limit oral magnesium absorption. In some people, parenteral replacement is needed until the underlying condition is controlled.
Raised magnesium in kidney failure
Cheryl had severe CKD (eGFR 14). She took magnesium hydroxide for constipation. Magnesium 1.6, feeling drowsy and weak.
What the GP said: “Your kidneys can't clear magnesium properly, and the magnesium-containing laxative is building up. Stop it and we'll switch you to a kidney-safe laxative.”
What happened next: Off the magnesium laxative, her level came back to 0.9 within a week.
Hypermagnesaemia in CKD is almost always iatrogenic — magnesium-containing laxatives or antacids. Check the medicine list whenever magnesium is high.
Before you act on anything here
This page is general information, written to help you understand — it is not personal medical advice and it cannot tell you what is right for you. Talk to your own GP, pharmacist or nurse about your own health.
- Need help now but it's not an emergency? Call NHS 111 or use 111.nhs.uk.
- Emergency? Call 999 or go to A&E.