The Encyclopaedia Britannica explains that pepsin is most active in the stomach. Pepsin is no longer effective in the intestine, where gastric acids are neutralized. Pepsin's digestive power is most significant at the acidity levels of ordinary gastric juice.
Pepsin is an enzyme found in gastric juice. It digests proteins found in foodstuffs such as dairy products, eggs, meat and seeds. Pepsin is produced in the stomach, where hydrochloric acid combines with the inactive protein known as pepsinogen. Pepsin is also active in small amounts in the bloodstream, where it breaks down larger or partially undigested protein fragments that the small intestine might have absorbed.