Gemcitabine in Treating Patients With Metastatic Cancer of Unknown Primary

DISEASE CHARACTERISTICS: * Histologically confirmed poorly differentiated carcinoma or adenocarcinoma of unknown primary * Diagnosis based on biopsy and conventional imaging, including CT scans of the chest, abdomen, and pelvis with or without positron emission tomography (PET) scans or other specialized tests, performed within the past 4 weeks * Must not have any of the following clinical features: * Squamous cell carcinoma in the lymph nodes of the neck or inguinal nodes only * Women with axillary lymph node metastases only * Women with peritoneal carcinomatosis only * Well-differentiated neuroendocrine tumors * Poorly differentiated tumors with midline tumor or elevated human chorionic gonadotropin (HCG)/alpha-fetoprotein (AFP) * Men with adenocarcinoma and elevated prostate-specific antigen (PSA) * Measurable disease * No symptom emergency at the time of study entry including, but not limited to, the following: * Back pain with epidural cord compression * Large effusions causing distress * Hypercalcemia * Bowel obstruction * Very painful (worst pain 10/10) solitary bone metastases with impending fracture PATIENT CHARACTERISTICS: * Modified "Physical Well-Being" subscale of the FACT-G score ≥ 6 within the past week * Pain-intensity score ≥ 20 mm on the Memorial Pain Assessment Card OR receiving analgesics of ≥ 10 mg per day of oral morphine equivalent within the past week * ECOG performance status 1-2 * WBC ≥ 3,000/mm³ * Platelet count ≥ 100,0…