Brief Answer:
Generally safe and well tolerated

Detailed Answer:
Dear Ms , I have gone through your question and I understand your concerns but there is no need to worry as magnesium is generally well tolerated and is not expected to cause side effects . however over use can lead to Hypermagnesemia (signs include drowsiness, loss of reflexes, nausea, vomiting, flushing, thirst, hypotension, hypothermia, bradycardia, confusion, electrolyte and fluid abnormalities, slurred speech, muscle weakness, muscle paralysis, blurred or double vision, CNS depression, respiratory depression, respiratory paralysis, circulatory collapse, renal failure, coma, cardiac arrhythmias, cardiac arrest) and hypocalcemia with tetany secondary to hypermagnesemia
To avoid this you need to regularly monitor your blood magnesium level.
As any drug taken in form of tablets it can cause gastric upset , and it. an cause irritation at the injection site if taken in the form of injections.
I hope I answered your question.

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