A Study to Evaluate the Safety and Effectiveness of Fentanyl-TTS Compared to Weak Opioids in Patients With Moderate to Severe Chronic Cancer Pain Previously Treated With NSAIDS (Non-steroidal Anti-inflammatory Drugs).

Inclusion Criteria: * Patients with moderate to severe chronic pain due to the oncological condition, defined as VAS =\> 5, treated with first step drugs, either NSAIDs or acetaminophen or metamizole (the dose of first step drugs requiring a switch in treatment is left to the physician's discretion) * Patients with a documented histological diagnosis of cancer. Exclusion Criteria: * Patients whose pain has some neuropathic component * Patients who have been treated with opioids during the 2 months prior to study entry * Patients with dermatological disease, a history of allergy or hypersensitivity to fentanyl or to adhesive components preventing use of patches * Patients with a history of cardiac, CNS, or respiratory disease that prevents their participation in the study in the investigator's judgment * Patients with kidney or liver failure contraindicating use of opioids based on medical criterion * Pregnant women or women of childbearing age who do not use an effective contraceptive method throughout the study * Patients in whom surgery is planned during the study * Patients who are participating at the same time in another trial.