A healthy, balanced diet that includes fiber-rich foods like fruits, vegetables, and grains is a good source of vitamins and minerals such as magnesium. Dietary magnesium promotes health and does not pose a health risk. However, pharmacologic doses of magnesium such as those found in supplements can have adverse effects especially when taken in large amounts or when a health problem affects its use.
Upper Limits of Magnesium Intake
There is no upper limit for magnesium that comes from dietary sources; however, according to the Institute of Medicine, for magnesium supplements, one must take care to observe these tolerable upper limits for intake:
For children 1-3 years old – 65 mg/day
For children 4-8 years old – 110 mg/day
Older than 8 years to adulthood – 350 mg/day
Magnesium is therefore considered to be likely safe for most people when used correctly, in doses less than 350 mg/day for healthy adults.
Risks and Side Effects
Magnesium supplements may possibly be unsafe when taken in large doses or when used by people who have certain health problems.
Minor side effects of taking these supplements are common, and these include diarrhea and abdominal cramping, stomach upset, nausea, and vomiting.
Taking large doses of these supplements on a regular basis might cause a build-up of magnesium in the body, causing serious side effects like confusion, irregular heartbeats, low blood pressure, slowed breathing, coma, and death.
Special precautions and warnings are given to people who have kidney disease and kidney failure, since these conditions may cause failure to eliminate excess magnesium from the body even when these supplements are taken in regular doses. Although dietary magnesium may not harm them, taking extra magnesium can cause a build-up of these minerals to dangerous levels.
Another condition that requires precaution is for people who have heart blocks, a type of heart rhythm irregularity, since magnesium influences electrical conduction and contraction of the heart. People with other types of heart rate irregularities such as atrial fibrillation and bradycardia (excessively slow heart rate) should consult a doctor before taking magnesium supplements.
Aside from taking magnesium as a dietary supplement, people also take magnesium in the form of antacids to relieve indigestion or laxatives to relieve constipation. However, taking too much of these antacids and laxatives may also lead to magnesium toxicity.
Signs of magnesium toxicity may be similar to those of magnesium deficiency. These include irregular heartbeat, changes in mental status, loss of appetite, nausea, diarrhea, muscle weakness, extremely low blood pressure, and difficulty in breathing.
One must also be careful of the possible drug interactions between magnesium and these medications:
Some antibiotics (aminoglycosides) like amikacin (Amikin), gentamicin (Garamycin), kanamycin (Kantrex), streptomycin, and tobramycin (Nebcin) interact with magnesium because these substances, like magnesium, affect the muscles.
Magnesium may reduce the absorption and the effects of quinolone antibiotics such as ciprofloxacin (Cipro), enoxacin (Penetrex), norfloxacin (Chibroxin, Noroxin), and sparfloxacin (Zagam). To avoid this interaction magnesium and antibiotics should not be taken together. One may take the antibiotics at least 2 hours before, or 4 to 6 hours after taking magnesium supplements.
Magnesium decreases the amount of tetracycline antibiotics absorbed in the stomach by attaching to them, thereby decreasing their effectiveness. To avoid this interaction one may take magnesium 2 hours before or 4 hours after taking these antibiotics: demeclocycline (Declomycin), minocycline (Minocin), and tetracycline (Achromycin).
Magnesium can also decrease how much bisphosphate is absorbed by the body, thereby reducing their effects. To avoid this interaction one must take bisphosphonate at least two hours before magnesium or later in the day. Examples of bisphosphonates are alendronate (Fosamax), etidronate (Didronel), risedronate (Actonel), and tiludronate (Skelid).
Medications for high blood pressure, specifically calcium channel blockers, when taken with magnesium, might cause the blood pressure to drop too low, since magnesium also reduces blood pressure. These include nifedipine (Adalat, Procardia), verapamil (Isoptin, Verelan), diltiazem (Cardizem), isradipine (DynaCirc), felodipine (Plendil), and amlodipine (Norvasc). Consult a doctor for advice on taking magnesium and these medications.
Taking magnesium along with muscle relaxants can increase the risk of side effects of these medications, since magnesium also relaxes the muscles. Some muscle relaxants include carisoprodol (Soma), pipecuronium (Arduan), atracurium (Tracrium), pancuronium (Pavulon), and succinylcholine (Anectine).
Some potassium-sparing diuretics or "water pills" can increase the levels of magnesium in the body, thus increasing the risk for toxicity. These "water pills" include amiloride (Midamor), spironolactone (Aldactone), and triamterene (Dyrenium).
The safest way to maintain normal magnesium levels in the body is to consume a healthy diet that includes magnesium-rich foods such as vegetables and grains. People who have a condition that increases their risk for magnesium deficiency may prevent or treat the deficiency by taking supplemental doses of magnesium in the form of tablets, capsules, powders, or liquids.
Magnesium supplements in doses up to 350 mg/day in adults are likely to be safe. However, taking large doses increases one’s risk for side effects and toxicity. Certain health conditions and medications warrant medical advice regarding supplemental intake of magnesium, since these may also increase one’s risk for undesirable side effects.