Magnesium in drinking water is not bad for you. At the concentrations found in tap and mineral water, it’s safe for healthy adults and may even offer modest health benefits. The World Health Organization has reviewed the evidence and concluded that magnesium is “not of health concern at levels found in drinking-water,” declining to set a maximum limit.
Most tap water in the United States contains less than 20 mg/L of magnesium, though some sources reach 120 mg/L or higher. For context, the recommended daily intake of magnesium for adults is 310 to 420 mg depending on age and sex. So even at the high end, two liters of water would supply roughly 240 mg, well within normal dietary range.
How Much Magnesium Water Actually Provides
The amount of magnesium in your water depends entirely on where you live and what brand you drink. Tap, mineral, and bottled waters range from as little as 1 mg/L to over 120 mg/L. In most of the U.S., typical drinking water provides less than 10% of the daily recommended intake. But in areas with very hard water (water that’s naturally high in dissolved minerals), drinking two liters could supply over 50% of what you need each day.
Hard water gets its name from dissolved calcium and magnesium. Calcium usually dominates, with a typical calcium-to-magnesium ratio of about 5 to 1 in groundwater. If your water leaves white deposits on faucets or you notice it doesn’t lather soap well, you likely have hard water and are getting a meaningful dose of magnesium with every glass.
Your Body Absorbs It Just as Well as Food
One common assumption is that magnesium dissolved in water might not absorb as effectively as magnesium from food or supplements. A crossover study published in Food & Nutrition Research tested this directly, comparing magnesium absorption from four mineral waters with different mineral profiles against bread and a magnesium supplement. The result: no significant differences. Whether you get magnesium from water, whole grains, or a pill, your body takes it up at comparable rates.
This matters because many people fall short on magnesium. If your water happens to be mineral-rich, it’s quietly contributing to your intake every time you drink, cook pasta, or make coffee.
Potential Heart Health Benefits
Rather than being harmful, magnesium in water appears to be protective for the heart. A meta-analysis of 10 studies covering over 77,000 cases of coronary heart disease found that people with higher magnesium levels in their drinking water had an 11% lower risk of dying from heart disease compared to those with the lowest levels. The association was strongest for heart attacks specifically, where the risk reduction reached 19%.
Most of this evidence comes from European populations, particularly in Scandinavia, where groundwater mineral content varies widely between regions. The findings were consistent enough across case-control and cohort studies to suggest a real, if modest, benefit. Magnesium helps regulate heart rhythm and blood vessel function, which likely explains the connection.
When It Could Cause Digestive Issues
Magnesium does have a well-known laxative effect, but only at doses far higher than what you’d encounter in drinking water. Magnesium sulfate (Epsom salt) causes diarrhea at pharmacological doses, hundreds of milligrams per kilogram of body weight. In animal studies, soft stools appeared at around 450 mg/kg, and watery diarrhea at 2,000 mg/kg. These are doses you’d reach by deliberately drinking a concentrated Epsom salt solution, not by turning on the tap.
Even very hard water rarely exceeds 120 mg/L of magnesium. Drinking two liters of that water gives you 240 mg total, roughly the amount in a standard supplement. You’d need to consume many times that amount in a short period to experience a laxative effect.
The Upper Intake Limit Doesn’t Apply to Water
You may have seen that the tolerable upper intake level for supplemental magnesium is 350 mg per day for adults. That number sometimes causes confusion because it can look lower than the recommended daily intake of 310 to 420 mg. The key distinction: the 350 mg cap only applies to magnesium from supplements and medications. It does not include magnesium naturally present in food or water. The NIH makes this explicit in its guidance.
So the magnesium in your tap water, mineral water, or well water falls outside that upper limit entirely. It’s treated the same as magnesium from spinach or almonds: a natural dietary source with no established ceiling for healthy people.
One Group That Should Pay Attention
The one situation where magnesium intake of any kind requires caution is kidney disease. Healthy kidneys efficiently filter excess magnesium and excrete it through urine. When kidney function is significantly impaired, magnesium can accumulate in the blood, a condition called hypermagnesemia. This is primarily a concern for people with kidney failure who also take magnesium-containing medications or supplements.
For someone with advanced kidney disease, even the relatively small amounts of magnesium in very hard water could contribute to a cumulative problem, especially combined with dietary sources and medications. If you have chronic kidney disease, your nephrologist will likely monitor your magnesium levels and advise you on total intake from all sources, including water.
For everyone else, the magnesium in your water is a free, well-absorbed mineral that your body needs and that most people don’t get enough of. Hard water may taste different or leave mineral buildup on your fixtures, but from a health standpoint, it’s working in your favor.