Studies have shown that type 2 diabetic patients (T2DM) may have low serum magnesium levels leading to poor control and outcome of the disease. Supplementation with Mg might improve overall diabetic control and disease outcome. However, there is yet no consensus on whether the ionized (iMg) or the total Mg (tMg) level should be used as a basis to determine the status of Mg in the blood. Recently it was shown that iMg may correlate better with diabetes control than tMg. Therefore, Mg supplements to diabetic patients may improve their disease status. Unfortunately, and to the best of our knowledge all of the available trials on Mg supplementation guided by iMg levels were conducted on healthy volunteers rather than T2DM patients, and they were all for a short period of time (10 days to 10 weeks). Here we hypothesize that supplementations of T2DM patients with Mg based on serum iMg levels correlate better with diabetes control and prognosis. Such hypothesis is supported by a retrospective study that concluded that iMg correlate better with BP control than tMg. In addition, another trial that investigated the effects of three Mg dietary supplements; Mg oxide, Mg citrate and Mg carbonate on healthy female young adults, showed that only Mg oxide led to an increase in the levels of iMg and tMg concentrations when compared to baseline. Furthermore, a case-control study on older diabetic patients revealed a significant association between iMg and HbA1c. This study aims to investigate the effect of supplementing Mg oxide tablets versus placebo tablets guided by serum iMg levels in T2DM patients with or without hypomagnesemia on diabetic control and prognosis.