Gemcitabine With or Without Capecitabine in Treating Patients With Advanced Pancreatic Cancer

DISEASE CHARACTERISTICS: Histologically or cytologically confirmed primary inoperable or metastatic pancreatic adenocarcinoma No CNS metastases PATIENT CHARACTERISTICS: Age: Over 18 Performance status: Karnofsky 60-100% Life expectancy: Not specified Hematopoietic: WBC at least 3,500/mm^3 Platelet count at least 100,000/mm^3 Hemoglobin at least 10.0 g/dL Hepatic: Bilirubin no greater than 5 times normal AST/ALT no greater than 5 times normal Alkaline phosphatase no greater than 5 times normal Renal: Creatinine clearance at least 30 mL/min Gastrointestinal: No grade 2 or greater nausea or grade 1 or greater vomiting No medical condition that would interfere with taking oral medications or with gastrointestinal absorption (e.g., small bowel obstruction) Other: No prior unanticipated severe reaction to fluoropyrimidine therapy No known hypersensitivity to fluorouracil No known dihydropyrimidine dehydrogenase deficiency No active infection No other serious concurrent systemic disorders that would preclude study participation No other malignancy within the past 5 years except adequately treated carcinoma in situ of the cervix or basal cell skin cancer Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No prior capecitabine No prior chemotherapy for advanced pancreatic cancer At least 1 year since prior radiochemotherapy for pancreatic cancer Endocrine therapy: Not specified Radiotherapy: See Chemotherapy At least 1 year since prior adjuvant radiotherapy for pancreatic cancer No concurrent radiotherapy Surgery: Prior Whipple procedure or duodenal bypass allowed Other: At least 1 month since prior investigational agents No concurrent sorivudine or its chemically related analogues (e.g., brivudine) No other concurrent anticancer or investigational drugs