Calcium carbonate tablets sold as an antacid since the 1930s. Because the U. S. Food and Drug Administration approves calcium carbonate as a safe and reliable source of calcium, it is also labelled as a calcium supplement (and it is much cheaper than any calcium supplements I've seen on the vitamin shelf). In the U.S., it is actually the #1 most popular calcium supplement because of doctors' recommendations (not just for osteoporosis, but for pregnancy and premenstrual symptoms).