Even when the economy is in better shape, acid indigestion and heartburn plague about a third of the U.S. population. About 100 million Americans experience heartburn every month; about 15 million battle it at least once a day.

Acid indigestion involves an excess of hydrochloric acid in the stomach. Its symptoms can include discomfort or a burning sensation in the upper portion of the stomach, nausea, abdominal bloating, or belching. Frequent indigestion can lead to irritation of the duodenum or to an irritation of the lining of the stomach, both of which can play a role in the development of peptic ulcer disease, which can be life-threatening.

Heartburn is a painful burning sensation in the esophagus, just below the breastbone, caused by stomach (gastric) acid flowing back into the esophagus. The pain often rises in the chest and may radiate to the neck or throat. Heartburn may be a symptom of gastroesophageal reflux disease (GERD) — injury to the esophagus that develops from chronic exposure of the esophagus to acid coming up from the stomach.

With more than $10 billion spent worldwide each year on antacids, pharmacists can expect to field any number of patient questions and concerns. Here are some cautions to keep in mind:

• Many brands contain more than one active ingredient, so it is important for patients to read the labels if they are shopping without their doctors' recommendations. (Studies have shown that most people with heartburn and indigestion do not seek medical attention.)
• Patients who have recently suffered massive upper GI hemorrhage should not take antacids containing aluminum hydroxide without consulting their doctor.
• Patients who have impaired kidney function should not take antacids containing magnesium hydroxide without their doctors' knowledge.
• Extended heavy use of antacids that contain calcium (20 grams or more daily for a prolonged period) may cause excess calcium in the blood, which can lead to kidney stones and reduced kidney function. People who already have impaired kidneys may develop milk-alkali syndrome (symptoms include nausea, vomiting, mental confusion, and loss of appetite) when taking as little as 4 grams a day of these antacids.
• Overuse of antacids that contain aluminum can weaken bones, especially in people with impaired kidney function, leading to osteomalacia (softening of the bones, which causes tenderness, muscular weakness, and weight loss).
• The most common side effect of antacids is a chalky taste left in the mouth. Antacids containing magnesium may cause diarrhea. Those containing aluminum may cause constipation. Taking antacids containing both aluminum and magnesium (e.g., Mylanta or Maalox) may balance these effects. Prolonged use of antacids may result in nausea, headache, muscle weakness, or loss of appetite.
• Antacids can affect the way a number of prescription medicines work or are used by the body. Finally, with many medications, antacids must be taken one to two hours before or after the other drug.