If you take magnesium oxide to help with leg cramps, does it make your muscles weak?

Yes, if take too muc: Magnesium oxide 420mg once/twice daily should not make muscles weak but if you take large amounts and have kidney failure, your magnesium level may get very high and this may be dangerous / cause weakness. It is possible that 420mg once daily could cause symptoms of weakness, but I have not seen this.

Should not: Mgo is used for relief of indigestion, heartburn as an antacid; short term laxative; and supplement. Side effects may be nausea, abdominal cramping, and long term use may cause bowel obstruction. It should not result in muscle weakness.