Final answer:
The major adverse effect of calcium antacids is constipation, and they may also contribute to kidney stones and hypercalcemia.
Step-by-step explanation:
The major adverse effect of calcium antacids is the potential to cause constipation. Calcium-containing antacids such as calcium carbonate, used to neutralize stomach acid, can lead to this side effect. Additionally, they are not recommended for individuals prone to kidney stones, as these are often composed of calcium salts.
Excessive use of calcium antacids may also result in an imbalance of calcium in the body, leading to hypercalcemia, a condition with symptoms including lethargy, sluggish reflexes, loss of appetite, confusion, and in severe cases, coma.