This pilot study will investigate the impact of dexamethasone (DEX) on anti-Xa levels in participants taking apixaban 2.5 mg twice a day by mouth (PO BID). Investigators propose a prospective, cohort study of 24 participants with multiple myeloma, in whom a lenalidomide-dexamethasone (LEN-DEX)-based myeloma treatment regimen is indicated. Eligible participants will initiate thromboprophylaxis with apixaban prior to starting their DEX-containing regimen and continue until the end of cycle 3. Anti-Xa levels, D-Dimer and plasma drug concentration will be measured.