Magnesium is an essential mineral for the body and brain, which is needed for the proper functioning of many enzymes that carry out biochemical reactions. Sufficient levels of magnesium are usually obtained through a healthy diet. Although preliminary research suggests that magnesium through diet or supplements might promote cognition and prevent cognitive decline, the evidence is very limited. Magnesium supplements are safe for most people, though they pose safety risks for people on certain medications.
Very few clinical trials have examined the role of magnesium in brain health and dementia prevention. Our search identified:
• 0 meta-analyses or systematic reviews
• 1 randomized controlled trial on cognition in older adults
• 2 observational studies on the risk of cognitive decline
• Multiple preclinical studies
People who consume diets rich in magnesium have been reported to have a lower risk of cognitive decline in two observational studies. Specifically, one study followed more than 1,400 healthy older adults for 8 years and reported that higher dietary magnesium intake was associated with an 86 percent reduced risk of developing mild cognitive impairment for men but not women . Another 17-year study that followed more than 1,000 Japanese adults over the age of 60 found that those who consumed more than 200 mg of magnesium per day were 37 percent less likely to develop any type of dementia and 74 percent less likely to develop vascular dementia . However, these studies do not prove that magnesium caused the reduced risk. For example, it is quite possible that other aspects of diet were responsible for the protective association.
No clinical trials have tested whether magnesium supplements or even a magnesium-rich diet can prevent dementia or cognitive impairment, but one pilot randomized controlled trial of 44 patients reported that magnesium L-threonate improved overall cognitive ability for elderly patients with memory complaints . Preclinical studies have provided a scientific rationale through which these benefits might occur, such as a decrease in the plaques and tangles of Alzheimer's disease (i.e., beta-amyloid and tau phosphorylation) as well as reduced brain inflammation, the generation of new neurons, and stronger connections between neurons [4-11]. Most of these studies used a recently developed magnesium supplement called magnesium threonate, but it remains to be proven whether these biological effects will occur in humans.
While it has been theorized that magnesium may benefit patients with dementia  and some preclinical evidence suggests that it may disrupt Alzheimer's progression (see above), there is little direct clinical evidence to support this. A small clinical trial (20 patients) is currently recruiting patients with mild to moderate dementia to test the benefits of 60 days of magnesium threonate supplementation; results are expected in 2016 .
Moderate use of magnesium supplements is considered safe for most people. The U.S. recommended dietary allowance for magnesium is 310 to 320 mg/day for women and 400 to 420 mg/day for men. Common side effects include nausea, vomiting, and diarrhea. Magnesium sulfates used with aminoglycoside antibiotics can increase the risk of muscle paralysis. Magnesium may also interact with bisphosphates, diuretics, and proton pump inhibitors. Many magnesium supplements may decrease the effectiveness of dolutegravir (Tivicay™), a drug commonly prescribed to HIV patients .
NOTE: This is not a comprehensive safety evaluation or complete list of potentially harmful drug interactions. It is important to discuss safety issues with your physician before taking any new supplement or medication.
Many foods are good sources of magnesium, including spinach, almonds, cashews, soy and black beans, yogurt, brown rice, and avocados. Oral magnesium supplements—typically classified as magnesium salts, acids, or amino acids—are commercially available. Some forms of magnesium, such as magnesium aspartate, citrate, lactate and chloride are more bioavailable than other forms such as magnesium oxide and sulfate . Most research on brain health has used the amino acid magnesium threonate. The U.S. recommended dietary allowance for magnesium is 310 to 320 mg per day for women and 400 to 420 mg per day for men.
Magnesium: Fact Sheet for Health Professionals and Magnesium in Diet from the National Institutes of Health
Drugs Interaction Checker from Drugs.com