Omeprazole 40 milligrams is typically prescribed to treat symptoms of excess stomach acid production such as gastroesophageal reflux disease, according to Drugs.com. Omeprazole belongs to a class of drugs called proton pump inhibitors, which block acid production in the stomach.
Omeprazole is not for immediate relief of heartburn, but rather for long-term reduction of stomach acid to reduce heartburn symptoms and allow healing of acid-related damage such as erosive esophagitis or gastric ulcers, according to Drugs.com. The dosage of omeprazole depends on the condition being treated, the patient's clinical response and tolerance of side effects. Common side effects include fever, cold symptoms, stomach pain, nausea and vomiting, according to RXList.com.