In the past decade we have seen four new agents approved by the US Food and Drug 
Administration for treatment of multiple myeloma: the proteasome inhibitor (PI) 
bortezomib (Velcade), the immunomodulatory agents lenalidomide (Revlimid) and 
thalidomide (Thalomid), and liposomal doxorubicin. These are commonly used in 
the treatment of relapsed/refractory (R/R) multiple myeloma (MM), but there is 
no universally accepted standard treatment. Salvage therapy must be tailored 
according to an individual patient's clinical profile, with the risks and 
potential effects of treatment-related adverse events being major determinants 
of the choice of therapy. Two novel agents in phase II studies to investigate 
their potential for the treatment of R/R MM are carfilzomib, a selective, 
irreversible next-generation PI, and pomalidomide, a next-generation thalidomide 
analog. This review will discuss the side-effect profiles of the currently 
approved immunomodulatory agents and bortezomib, as well as those of the newer 
agents, carfilzomib and pomalidomide.