The purpose of this study is to determine the effectiveness and safety of sublingual fentanyl tablets in relieving breakthrough pain in cancer patients.

Inclusion Criteria:

• Males or females 17 years of age or older
• Stable cancer-related pain.
• Are receiving a stable, fixed-schedule oral opioid regimen equivalent to 60 to 1000 mg of oral morphine per day or transdermal fentanyl therapy equivalent to 50 to 300 µg/h, and are on a stable dose of opioid medication for relief of breakthrough pain for at least 14 days prior to screening.
• Experiencing 1-4 episodes of breakthrough pain per day.
• Meet the criteria defined in the Eastern Cooperative Oncology Group (ECOG) Performance Status for Grade 0, 1, or 2.

Exclusion Criteria:

• Are pregnant or lactating.
• Have uncontrolled or rapidly escalating pain.
• Have moderate to severe ulcerative mucositis.
• Have a cardiopulmonary disease that would increase the risk of administering potent opioids.
• Have neurologic or psychologic disease that would compromise data collection
• Have any clinically significant condition that would, in the investigator's opinion, preclude study participation.
• Are currently taking monoamine oxidase inhibitors (MAOIs), or have taken MAOIs within 14 days prior to enrolling in the study.
• Have received strontium 89 therapy within 60 days prior to entering the study.
• Have received anti-neoplastic therapy within 2 weeks of study entry that, in the investigator's opinion, will influence assessment of breakthrough pain
• Have received any investigational drug (non-approved) within 30 days prior to the first dose of study medication, or are scheduled to receive an investigational drug other than EN3267 during the course of the study.
• Have hypersensitivities, allergies, or contraindications to fentanyl.
• Have a significant prior history of substance abuse or alcohol abuse.
• May have difficulty complying with the protocol, as assessed by the investigator.