PATIENT CHARACTERISTICS: Age: 21 and over Performance status: ECOG 0-2 Life expectancy: Not specified Hematopoietic: Platelet count greater than 100,000/mm3 Hemoglobin greater than 10 g/dL No untreated pernicious anemia Hepatic: Bilirubin less than 2 mg/dL ALT less than 2 times normal Alkaline phosphatase less than 2 times normal PT normal Renal: Not specified Gastrointestinal: No intestinal malabsorption No inflammatory bowel disease Other: No seizure within the past year No poor medical risk No other malignancy within past 5 years except basal cell cancer, superficial skin cancer, or carcinoma in situ of the cervix No vitamin B12 deficiency (less than 200 pg/mL) Not pregnant or nursing Fertile patients must use effective barrier contraception