DISEASE CHARACTERISTICS:

• Diagnosis of multiple myeloma (MM) based on one of the following:

  • Bone marrow plasmacytosis with at least 10% abnormal plasma cells
  • Multiple biopsy-proven plasmacytomas

• At least 1 of the following required:

  • Myeloma protein in serum
  • Myeloma protein in urine, i.e., free monoclonal light chain

  • Radiologic evidence of osteolytic lesions

    • Generalized osteoporosis qualifies only if bone marrow aspirate contains at least 20% plasma cells
• No smoldering myeloma

• Planning to initiate 1 of the following regimens as primary therapy for MM within 3 days of study entry:

  • Myelosuppressive chemotherapy
  • High-dose dexamethasone
  • Dexamethasone and thalidomide

PATIENT CHARACTERISTICS:

Age:

• 18 and over

Performance status:

• Not specified

Hematopoietic:

• Not specified

Hepatic:

• Not specified

Renal:

• Creatinine less than 5.0 mg/dL

• No requirement for dialysis at study entry

  • If required after entry, patients continue study with adjusted medication guidelines

Other:

• Not pregnant
• No history of hypersensitivity to fluoroquinolones or trimethoprim
• At least 7 days since prior active infection

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• See Disease Characteristics
• No bone marrow transplant or autologous stem cell rescue planned within first 2 months of myeloma chemotherapy
• No concurrent prophylactic filgrastim (G-CSF) during the first 2 months of study participation
• No concurrent intravenous immunoglobulins

Chemotherapy:

• See Disease Characteristics
• No prior chemotherapy (except mithramycin)

Endocrine therapy:

• See Disease Characteristics
• Prior corticosteroids allowed
• No prior high-dose dexamethasone

Radiotherapy:

• At least 10 days since prior radiotherapy
• No radiotherapy planned for near future

Surgery:

• Not specified

Other:

• At least 7 days since prior antibiotics
• No concurrent theophylline
• No concurrent sucralfate or oral antacids if receive ciprofloxacin or ofloxacin