Capecitabine in Treating Patients With Metastatic Breast Cancer

DISEASE CHARACTERISTICS: * Histologically or cytologically confirmed diagnosis of adenocarcinoma of the breast * Evidence of metastatic involvement (stage IV disease) * Patients must have measurable disease * At least one measurable lesion as defined by the Response Evaluation Criteria in Solid Tumors (RECIST) * Treated brain metastases (surgery or radiation therapy) allowed if clinically stable * Patients with leptomeningeal disease are ineligible * Hormone receptor status: * Not specified PATIENT CHARACTERISTICS: * Eastern Cooperative Oncology Group (ECOG) performance status 0-2 * Male or female * Menopausal status not specified * Absolute neutrophil count (ANC) ≥ 1,500/mm\^3 * Platelet count ≥ 100,000/mm\^3 * Creatinine clearance \> 50 mL/min * Fertile patients must use effective contraception * No history of another severe and/or life-threatening medical disease * No other active primary malignancy * Not pregnant or nursing * Negative pregnancy test * Patients with asymptomatic HIV infection are eligible * Liver dysfunction score ≤ 9 * No pre-existing liver disease (i.e., cirrhosis or active viral hepatitis) * No active gastrointestinal malabsorption illness * No clinically significant cardiac disease, including the following: * Congestive heart failure, symptomatic coronary artery disease, and cardiac arrhythmias not well controlled with medication, or myocardial infarction within the past six months * No prior unanticipated severe reaction to fluoropyrimidine therapy, known hypersensitivity to fluorouracil, or known dihydropyrimidine dehydrogenase deficiency * No history of uncontrolled seizures or central nervous system disorders * No significant history of noncompliance to medical regimens * No clinically significant psychiatric disability that would preclude study compliance PRIOR CONCURRENT THERAPY: * No previous capecitabine * Up to 3 prior cytotoxic regimens allowed for metastatic disease * Prior noncytotoxic therapy allowed (e.g., hormonal treatment or trastuzumab) * No other concurrent therapies intended to treat the primary condition including chemotherapy, biologic agents, or immunotherapy * No concurrent anti-estrogen therapy, radiation therapy, or investigational systemic therapy * No other concurrent investigational drugs * No concurrent use of the following drugs: warfarin for full anticoagulation, cimetidine, or azidothymidine (AZT) * Mini-dose warfarin for prophylaxis of central venous catheter thrombosis allowed * At least 4 weeks since prior sorivudine or brivudine * Concurrent use of bisphosphonates allowed if initiated before beginning study therapy * Concurrent use of megestrol acetate suspension as an appetite stimulant allowed