Treatment of cancer pain is based on the World Health Organization (WHO) step ladder approach to the use of analgesic drugs. Medication potency increases at each step of the WHO ladder, from nonopioid (step 1; e.g., aspirin and nonsteroidal anti-inflammatory drugs) through weak opioids (step 2; e.g. codeine) plus a nonopioid to strong opioids (step 3; e.g., morphine) plus a nonopioid analgesic. Morphine is considered the gold standard for the treatment of moderate to severe pain, but this is based on level C criteria. Research has discovered that methadone is a potent opioid that operates at several levels which are important for pain control. The primary aim is to compare morphine versus methadone as a first-line analgesic in patients with moderate to severe cancer pain. Patients will be randomized to receive either oral slow-release morphine (15 mg) every 8 hours and immediate-release morphine (10 mg) every 4 hours as needed for breakthrough pain or oral methadone 2.5 mg every 8 hours and methadone 2.5 mg every 4 hours as needed for breakthrough pain. Our hypothesis is that methadone will provide equivalent pain control efficacy after 4 weeks of therapy. We postulate that methadone will be as preferable as morphine as an analgesic. We will compare the two drugs via adverse effects and compare stability of analgesia via comparison of the number of breakthrough pain episodes. The study will attempt to establish equivalency of methadone as a first-line analgesic for moderate to severe cancer pain.