Simplify the following text for a Grade 9 native English-speaking person.
Original: Calcium carbonate is used as a calcium supplement .
Simplified: Acute calcium poisoning is rare , and difficult to achieve unless calcium compounds are administered intravenously . For example , the oral median lethal dose ( LD50 ) for rats for calcium carbonate and calcium chloride are 6.45 and 1.4 g\/kg , respectively .