Current guidelines on the treatment of moderate to severe cancer pain recommend the use of scheduled doses of opioids for persistent pain combined with "as needed" doses of similar agents for breakthrough pain. Oral drugs given on an "as needed" basis can be problematic for patients with difficulty in swallowing or for those who suffer from nausea and vomiting. Further, breakthrough pain can become excruciating in a relatively short time, a drawback for analgesics that require gastrointestinal (GI) absorption before pain relief can begin. Hence, there is considerable interest in the development of novel drug administration routes to provide rapid relief of breakthrough pain, particularly through a route that bypasses the GI system. Sublingually administered morphine has sometimes been used in the treatment of breakthrough pain because some believe it provides effective analgesia via an appropriate alternate route. Available pharmacological data, however, do not consistently support the rapid absorption of morphine through the sublingual mucosa, and clinical data concerning the efficacy of sublingual morphine for the treatment of cancer pain are limited, not well-controlled, and inconclusive. While there seems to be a need for provision of rapid, effective analgesia to cancer patients by an alternative route, sublingual morphine may not satisfy this requirement. Newer formulations of analgesics should be tested in the treatment of breakthrough pain due to cancer.