Can I take 500mg of magnesium oxide with a daily multivitamin? I also take atenolol for fast heart rate? Please advise

Yes, but poor absorb. Yes, you can, but magnesium oxide is only about 4% absorbed so it is much more likely to act as a laxative than raise your magnesium level! i advise taking magnesium taurate or glycinate with at least 375 mg of mag./day. Putting 2 cups of epsom salts into a bath and soaking 20 min. Or topical "magnesium oil" also increases magnesium levels. See http://bit.Ly/1dpai09 also eat magnesium-rich foods!