The pathophysiology behind chronic gastric and duodenal ulcer is not fully understood, but may be explained as an imbalance between aggressive and defensive factors acting on or in the mucosa. Medical therapy may aim to reduce the aggressive factors acid and pepsin by compounds that neutralize the acid or inhibit the secretion of acid. Peptic activity is reduced by the high acid pH and also by the inhibition. Another group of compounds increases the mucosal defence by stimulating the secretion of mucus, bicarbonate, and growth of the mucosa, or by forming a protective layer on the ulcer crater. Some compounds affect both the aggressive and defensive factors. All modern compounds heal most ulcers in 4 to 6 weeks, but do not alter the natural history of ulcer disease, as indicated by the high rate of ulcer relapses after cessation of treatment.