Gastroduodenal mucosa has a self-defense capacity against a wide range of potentially harmful exogenous and endogenous agents. It has been proven that certain diet compounds damage gastric mucosa, which explains--at lest partially--the regional variations in the incidence of peptic ulcer. Ethanol blocks the defense mechanisms of gastric mucosa and induces the onset of acute lesions, but there is no definite proof to show that ethanol ingestion helps produce the onset of peptic ulcer. It has been confirmed that tobacco negatively affects the healing and relapse of ulcers. From an epidemiological point of view, the controversy of the relationship between tobacco abuse and peptic ulcer genesis is still ongoing. Non-steroid anti-inflammatory non-acid soluble produces lesions acute and chronic) in gastroduodenal mucosa. They can reactivate old lesions and increased the risk of complications. Aspirin is the most harmful compound in this pharmacologic group. Paracetamol is the compound which has the safest spectrum. The harmfulness of steroids is still being discussed, however, it is accepted that they have a rapid effect on gastric mucosa in relation to dosage and duration of treatment.