The side effects of magnesium commonly include upset stomach and diarrhea.
More Info: It is rare to get too much magnesium from diet alone. In fact, most people do not get enough magnesium in their diets. If an overdose does occur, it is generally attributed to magnesium supplementation, which should never be taken without a physician's supervision. 
Benefits of Magnesium
Magnesium is a necessary mineral in the human body and aids in the production of healthy cells, proper digestion, and overall good health. The best way to boost magnesium levels in your body is to change your diet to eat at least five servings of fruits and vegetables every day. Dark, leafy vegetables are recommended by the Food Guide Pyramid to help adults who are risk of having a magnesium deficiency. 
Who Is at Risk for a Magnesium Deficiency?
Though the general population does not get enough daily magnesium through diet alone, the signs of deficiency are usually rare because the body stores the mineral.  Excessive consumption of alcohol, soda, coffee, and salt can cause a magnesium deficiency. Some medications can cause a deficiency of magnesium in the body such as certain diuretics, chemotherapy drugs, certain antibiotics, steroids, and digoxin. Certain medical conditions can place you at risk for a serious magnesium deficiency such as kidney problems, diabetes, hyperthyroidism, pancreatitis, and gastrointestinal diseases. 
More Serious Side Effects of Too Much Magnesium
Though magnesium absorbed through diet poses no health risk, pharmacological supplementation can, such as that found in laxatives and antacids that contain magnesium. 
Too much magnesium can cause serious side effects and can even be fatal. Side effects can include nausea and vomiting, slowed heart rate, dangerously low blood pressure, confusion, respiratory paralysis, cardiac arrhythmias, coma, cardiac arrest, and death. Magnesium supplementation should always be monitored by a physician.