High magnesium in the blood, called hypermagnesemia, is almost always caused by either kidney problems that prevent your body from excreting magnesium or excessive intake of magnesium-containing products like laxatives, antacids, or supplements. Normal serum magnesium falls between 1.7 and 2.4 mg/dL, and levels above that range can start causing symptoms ranging from mild nausea to, in rare extreme cases, cardiac arrest.
Kidney Disease Is the Most Common Cause
Your kidneys are the primary way your body gets rid of excess magnesium. When kidney function declines, magnesium builds up in the blood because it simply has nowhere to go. The kidneys try to compensate by reabsorbing less magnesium in the tubules, but this workaround has limits. Once kidney disease reaches stage 4 (meaning the kidneys are filtering at roughly 15 to 29% of normal capacity), that compensation can no longer keep up, and magnesium levels start to rise.
This is why hypermagnesemia is relatively rare in people with healthy kidneys. Even if you take in a large amount of magnesium, functional kidneys can usually flush the excess. But for people with chronic kidney disease or end-stage kidney failure, even a normal dietary intake of magnesium can push blood levels higher than they should be.
Over-the-Counter Products That Contain Magnesium
Many common drugstore products contain surprisingly high amounts of magnesium. These include antacids (like milk of magnesia), osmotic laxatives, and magnesium supplements. Magnesium toxicity is more common in the United States than in many other countries, likely because these products are so widely available without a prescription.
For someone with healthy kidneys, occasional use of a magnesium-containing antacid or laxative is unlikely to cause problems. The danger comes from heavy or prolonged use, especially in older adults whose kidney function may have quietly declined. A person who takes magnesium laxatives daily for chronic constipation while also using a magnesium-based antacid for heartburn can accumulate a significant load without realizing it. When reduced kidney function is added to the equation, the risk jumps considerably.
Magnesium Given in Hospital Settings
Intravenous magnesium sulfate is a standard treatment for preeclampsia and eclampsia during pregnancy, as well as for certain heart rhythm problems. The therapeutic target for preeclampsia is intentionally above the normal range, sitting between 4 and 7 mEq/L. Because this window borders on toxic levels, medical teams monitor blood levels every 6 to 8 hours and watch for early warning signs like loss of reflexes in the knee. Despite close monitoring, hospital-administered magnesium remains a recognized cause of hypermagnesemia, particularly if kidney function drops unexpectedly during treatment.
Endocrine and Hormonal Conditions
Several hormonal disorders can raise magnesium levels, though the mechanisms are not fully understood for all of them.
- Adrenal insufficiency (Addison’s disease): Low aldosterone levels cause the body to lose sodium and water, which concentrates the blood and raises the apparent level of magnesium. The reduced blood flow to the kidneys also slows magnesium excretion.
- Hypothyroidism: Thyroid hormones influence both kidney blood flow and the rate at which the kidneys reabsorb minerals, including magnesium. When thyroid function drops, the kidneys handle magnesium differently, and levels can creep upward.
- Hyperparathyroidism: Overactive parathyroid glands alter calcium and magnesium handling in the kidneys, potentially raising blood magnesium.
A Rare Genetic Cause
Familial hypocalciuric hypercalcemia (FHH) is an inherited condition caused by mutations in the gene for the calcium-sensing receptor. This receptor normally tells the kidneys when calcium and magnesium levels are high enough and it’s time to stop reabsorbing them. When the receptor is less sensitive due to the mutation, the kidneys keep pulling calcium and magnesium back into the blood instead of excreting them. People with FHH typically have mildly elevated or high-normal magnesium levels alongside elevated calcium. It’s usually benign and often discovered incidentally on blood work.
Symptoms at Different Magnesium Levels
Hypermagnesemia tends to produce symptoms in a dose-dependent way, meaning the higher the level, the more serious the effects. At mildly elevated levels (below 7 mg/dL), you might feel weak, nauseated, dizzy, or mentally foggy, or you might not notice anything at all. Many cases of mild hypermagnesemia are caught on routine blood work before symptoms appear.
As levels climb toward 12 mg/dL, deep tendon reflexes disappear. This is something a clinician checks by tapping below the kneecap: if the leg doesn’t kick, magnesium is likely dangerously high. At these levels, blood pressure drops, breathing slows, muscles become limp, and severe drowsiness or lethargy sets in.
Above 15 mg/dL, cardiac arrest becomes a real risk. Coma and complete respiratory failure can occur. These extreme levels are rare and almost exclusively seen in people with significant kidney impairment who are also taking in large amounts of magnesium, or in cases of accidental overdose in a medical setting.
Who Is Most at Risk
The typical person who develops hypermagnesemia is an older adult with chronic kidney disease who regularly uses magnesium-containing laxatives or antacids. Age-related decline in kidney function often goes undiagnosed, meaning someone may not realize their kidneys can no longer handle the magnesium load from products they’ve used for years without issue.
People receiving IV magnesium in a hospital, particularly pregnant women being treated for preeclampsia, are monitored precisely because the therapeutic dose sits close to the toxic threshold. And anyone with an endocrine disorder affecting the adrenal glands, thyroid, or parathyroid glands has a modestly increased baseline risk, though clinically significant hypermagnesemia from these conditions alone is uncommon.
If you have kidney disease at any stage, checking the labels on antacids, laxatives, and supplements for magnesium content is a practical step that can prevent an avoidable problem.