Causes of stomach upset caused by taking calcium tablets include drug stimulation, indigestion, allergy and so on. 1. Drug stimulation: Calcium tablets contain a lot of alkaline substances in addition to calcium content, calcium tablets will change the acid-base environment in the stomach after being digested, affecting the gastric mucosa and function, resulting in stomach discomfort. 2. Indigestion: When patients suffer from indigestion due to various reasons, calcium tablets are not easily digested by the stomach, which increases the burden on the stomach, and patients will suffer from abdominal discomfort such as bloating. 3. Allergy: due to the complexity of the types of calcium tablets, containing a variety of ingredients, if the patient of some of the ingredients in the calcium tablets produce a perverse reaction, causing gastric mucosal congestion and edema degeneration, stimulation of the gastric mucosa, abdominal pain and other stomach discomfort will occur. There are many reasons for stomach discomfort caused by taking calcium tablets, if patients still have discomfort after stopping calcium tablets, it is recommended to go to the hospital under the guidance of a doctor.