Hi, I am Dr. Michael Murray. Many headaches may be a sign of magnesium deficiency. For example, studies show 30 to 50 percent of people with migraine headaches are deficient in magnesium. Magnesium deficiency is suspected to cause blood-vessel spasms that lead to a headache. Studies have shown that migraine sufferers with low magnesium levels who take 400 to 800 mg of magnesium daily can have a significant decrease in the frequency of migraine headaches after three to four weeks of supplementation.