Inclusion Criteria

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