doses of milk of magnesia, and in the other case, several doses of magnesium sulfate in a patient with mild renal impairment (Golzarian et al., 1994). Cardiorespiratory arrest was encountered in a suicidal patient given 465 g (19.1 mol) of magnesium sulfate as a cathartic to counteract an intentional drug overdose (Smilkstein et al., 1988). Deaths from very large exposures to magnesium in the form of magnesium sulfate or magnesium oxide have been reported following cardiac arrest, especially in individuals with renal insufficiency (Randall et al., 1964; Thatcher and Rock, 1928).

Data Selection. A review of the scientific literature revealed relatively few reports that were useful in establishing a UL for magnesium. Because magnesium has not been shown to produce any toxic effects when ingested as a naturally occurring substance in foods, a UL cannot be established for dietary magnesium at this time. In addition, studies involving intravenous administration of comparatively large doses of magnesium used in the treatment of preterm labor, pregnancy-induced hypertension, or other clinical conditions were not considered applicable for the derivation of ULs. Based on limited data described below, a UL can be established for magnesium from nonfood sources.

Identification of a NOAEL (or LOAEL) and Critical Endpoint. As the primary initial manifestation of excessive magnesium intake, diarrhea was selected as the critical endpoint. The few studies that report mild diarrhea and other gastrointestinal symptoms from uses of magnesium salts were reviewed to identify a No-Observed-Adverse-Effect Level (NOAEL) (or Lowest-Observed-Adverse-EffectLevel [LOAEL]). Gastrointestinal symptoms, including diarrhea, developed in 6 of 21 patients (51-to 70-year-old males and females) receiving long-term magnesium chloride therapy at levels of 360 mg (15 mmol) of magnesium (Bashir et al., 1993). Gastrointestinal manifestations developed in 5 of 25 pregnant women being given 384 mg (16 mmol) of daily magnesium as magnesium chloride supplements for the prevention of preterm delivery, although one patient receiving the placebo treatment also developed diarrhea (Ricci et al., 1991). Diarrhea was also noted in 18 of 50 healthy white and black men and women (aged 31 through 50 years) who were ingesting 470 mg (19.6 mmol) of magnesium as magnesium oxide