Inclusion Criteria:  cancer patients regularly experienced 1 to 3 breakthrough pain episodes per day that required additional opioids from pain control; a current regimen of opioids equivalent to 60-360 mg/day of oral morphine or 25-150 mcg/hr of transdermal fentanyl for one week or longer; at least partial relief of breakthrough pain by use of opioid therapy; able to self-administer the study medication correctly or has an available adult caregiver to administer the study medication correctly; willing and able to complete patient diary with each pain episode. 20 to 80 years of age;  Exclusion Criteria:  rapidly escalating pain (e.g., regularly more than 3 breakthrough pain episodes per day) that are hard to be controlled by analgesics; history of hypersensitivity or intolerance to fentanyl; cardiopulmonary disease that, in the opinion of the investigator, would significantly increase the risk of respiratory depression; psychiatric/cognitive or neurological impairment that would limit the subject's ability to understand or complete the diary; Severe (Grade 4) mucositis (subjects with less than severe mucositis are permitted and must be instructed to not apply the Painkyl® film at a site of inflammation); abnormal oral mucosa which will impede drug absorption; recent history or current evidence of alcohol or other drug substance (licit or illicit) abuse; use of an investigational drug or other rapid-onset opioids drugs within 4 weeks preceding this study; pregnant women or nursing mothers, or positive pregnancy test for women of childbearing potential.