DISEASE CHARACTERISTICS:

• Histologically or cytologically proven advanced cancer with chronic pain
• Cancer pain requiring strong opioids having an average pain score of less than 6/10 on the visual analog scale within last 24 hours
• Pain managed by a stable maintenance dose of MS-Contin formulation of morphine for at least 2 days with no more than 2 breakthrough immediate release morphine doses per 24 hours
• Pain that is expected to be controlled by a stable and adequate total daily dose of sustained release morphine for the first 7 days of the study

PATIENT CHARACTERISTICS:

Age:

• 16 and over

Performance status:

• Not specified

Life expectancy:

• At least 2 months

Hematopoietic:

• Not specified

Hepatic:

• SGOT or SGPT no greater than 3 times upper limit of normal (ULN)
• No liver disease

Renal:

• Creatinine no greater than 2 times ULN
• No kidney failure

Pulmonary:

• No clinically significant respiratory depression
• No severe obstructive airway disease

Other:

• Fluent in English or French
• No known hypersensitivity or allergy to study medications or components or other multiple drug allergies
• Normal cognition defined by the Folstein Mini-Mental State Questionnaire (at least 24/30 correct)

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• Not specified

Chemotherapy:

• At least 14 days since prior chemotherapy

Endocrine therapy:

• Concurrent steroids allowed

Radiotherapy:

• At least 14 days since prior analgesic radiotherapy

Surgery:

• Not specified

Other:

• At least 3 months since prior investigational agents
• At least 1 month since prior clinical study
• No concurrent analgesics other than morphine
• No other concurrent medications containing dextromethorphan
• Concurrent antidepressant medication allowed
• Concurrent nonsteroidal antiinflammatory drugs allowed
• At least 14 days since prior monoamine oxidase (MAO) inhibitors
• No concurrent MAO inhibitors