As per Lu and Nightingale, serum magnesium concentrations associated with maternal toxicity (also neonate depression, hypotonia and low Apgar scores) are:

7.0–10.0 mEq/L – Loss of patellar reflex
10.0-13.0 mEq/L – Respiratory depression
15.0-25.0 mEq/L – Altered atrioventricular conduction and (further) complete heart block
>25.0 mEq/L – Cardiac arrest

Complications
Severe hypermagnesemia (levels greater than 12 mg/dL) can lead to cardiovascular complications (hypotension and arrhythmias) and neurological disorder (confusion and lethargy). Higher values of serum magnesium (exceeding 15 mg/dL) can induce cardiac arrest and coma. Causes
Magnesium status depends on three organs: uptake in the intestine, storage in the bone, and excretion in the kidneys. Hypermagnesemia is therefore often due to problems in these organs, mostly the intestine or kidney. Predisposing conditions
Hemolysis, magnesium concentration in red blood cells is approximately three times greater than in serum, therefore hemolysis can increase plasma magnesium. Hypermagnesemia is expected only in massive hemolysis. Chronic kidney disease, excretion of magnesium becomes impaired when creatinine clearance falls below 30 ml/min. However, hypermagnesemia is not a prominent feature of chronic kidney disease unless magnesium intake is increased. Magnesium toxicity from emergency pre-eclampsia treatment during labor and delivery. Other conditions that can predispose to mild hypermagnesemia are diabetic ketoacidosis, adrenal insufficiency, hypothyroidism, hyperparathyroidism, and lithium intoxication. Metabolism
For a detailed description of magnesium homeostasis and metabolism see hypomagnesemia. Diagnosis
Hypermagnesemia is diagnosed by measuring the concentration of magnesium in the blood. Concentrations of magnesium greater than 1.1 mmol/L are considered diagnostic.
Marketing
Methocarbamol without other ingredients is sold under the brand name Robaxin in the U.K., U.S., Canada and South Africa; it is marketed as Lumirelax in France, Ortoton in Germany and many other names worldwide. In combination with other active ingredients it is sold under other names: with acetaminophen (paracetamol), under trade names Robaxacet and Tylenol Body Pain Night; with ibuprofen as Robax Platinum; with acetylsalicylic acid as Robaxisal in the U.S. and Canada. However, in Spain the tradename Robaxisal is used for the paracetamol combination instead of Robaxacet. These combinations are also available from independent manufacturers under generic names. Research
Although opioids are a typically first line in treatment of severe pain, several trials suggest that methocarbamol may improve recovery and decrease hospital length of stay in those with muscles spasms associated with rib fractures. However, methocarbamol was less useful in the treatment of acute traumatic pain in general.Long-term studies evaluating the risk of development of cancer in using methocarbamol have not been performed. There are currently no studies evaluating the effect of methocarbamol on mutagenesis or fertility.The safety and efficacy of methocarbamol has not been established in pediatric individuals below the age of 16 except in tetanus. References
External links
"Methocarbamol". Drug Information Portal. U.S. National Library of Medicine.