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.
• Experiencing 1-4 episodes of breakthrough pain per day.
• Have a caregiver (e.g., spouse, sibling) living in the same household if the patient is not confined to a healthcare facility.
• Meet the criteria defined in the Eastern Cooperative Oncology Group (ECOG) Performance Status for Grade 0, 1, or 2.

Exclusion Criteria:

• Have previously been exposed to EN3267.
• Are pregnant or lactating.
• Have uncontrolled or rapidly escalating pain.
• Have any clinically significant condition that would, in the investigator's opinion, preclude participation in the study or compromise data collection. These conditions may include cardiopulmonary disease, and/or neurologic/psychologic conditions.
• Are scheduled to take MAOIs (monoamine oxidase inhibitors) during the study.
• Are scheduled to receive anti-neoplastic therapy that, in the investigator's opinion, will influence assessment of breakthrough pain.
• Are scheduled to receive an investigational drug other than EN3267 during the course of the study.
• Have hypersensitivity, allergy or contraindication to fentanyl.
• Have significant prior history of substance abuse or alcohol abuse.
• Would have difficulty complying with the protocol, as assessed by the investigator.
• Are unable to read, write, or comprehend the English language in order to complete diaries.