Final answer:
The earliest sign of magnesium toxicity can include gastrointestinal symptoms such as nausea, vomiting, and diarrhea.
Step-by-step explanation:
The earliest sign of magnesium toxicity can include gastrointestinal symptoms such as nausea, vomiting, and diarrhea. These symptoms may occur when there is an excessive intake of magnesium supplements or certain medications containing magnesium. In more severe cases, magnesium toxicity can lead to symptoms such as muscle weakness, difficulty breathing, and abnormal heart rhythms.