Hypermagnesemia occurs at serum magnesium levels over 25 mM (60 mg/dL)

ICD-9-CM designates diagnosis code 775.5 for Neonatal Hypermagnesemia. This code is found in the Index to Disease under Hypermagnesemia, see subterm neonatal 775.5. ICD-9-CM Chapter 3 Endocrine codes are excluded from assignment on a newborn's chart, when there is a specific code designated elsewhere in ICD-9-CM. This condition, Neonatal Hypermagnesemia does not require assignment of an additional code from Chapter 3. The Chapter 15 Neonatal code 775.5 is the designated code assignment.

Chronic alcoholism is the most common cause of hypomagnesemia, in part because of poor diet

When someone has too much of something in their body, the last thing you should do is put more in.

It's not exactly an "antidote", but the treatment for hypermagnesemia is calcium salts, IV saline, and possibly diuretics and/or dialysis.