Is it safe to take 5 or 6 500mg magnesium oxide vitamins everyday to relieve constipation/gastroinestinal issues?

If kidneys R normal. If your kidney function is normal, they will get rid any excess magnesium that you're taking. Don't take all 5 (2, 000 mg) at the same time - split them up. Also, the main side effect is diarrhea so you've been warned!