Significant preclinical and clinical research has explored the use of magnetic iron oxide nanoparticles (MNPs) for medical theranostics. Herein, we provide an overview of the optimal 'design-to-perform' MNPs used in cancer therapeutics, specifically focusing on magnetic hyperthermia, magnetic drug targeting, and targeting delivery. An account of the progress made in the clinic using MNPs is then analyzed. We place special emphasis on past and present magnetic nanoformulations used in clinical settings or yet to be clinically approved. Regrettably, as of now, no MNP drug delivery system is employed in the clinic. Thus, identifying current limitations, misconceptions and challenges will definitely impact the clinical success of MNP delivery theranostic systems and their promising future potential in medicine.