Antacids containing substances such as sodium bicarbonate, magnesium hydroxide, or calcium carbonate fundamentally are bases. They work by counteracting or neutralizing gastric acids to relieve the discomfort caused by gastric acidity. These medications typically are taken when gastric acidity is most likely to be increasing, such as within an hour or two after a meal and at bedtime.
Other antacids, such as histamine receptor antagonists (e.g., ranitidine and cimetidine) and proton-pump inhibitors (e.g., omeprazole), work by reducing acid secretion in the stomach. They do so by blocking the action of hormones on the acid-secreting parietal cells of the stomach. These drugs take additional time to work but have long-lasting effects relative to traditional antacids.