Nanoparticles of transition metal dichalcogenides, particularly of molybdenum (Mo), have gained a lot of focus due to their exceptional physicochemical properties and the growing number of technological applications. These nanoparticles are also considered as potential therapeutic tools, biosensors or drug carriers. It is crucial to thoroughly examine their biocompatibility and ensure safe usage. The aim of this review is to analyze the available data on the biological effects of different nanoforms of elemental Mo and its compounds. In the reviewed publications, different conditions were described, including different experimental models, examined nanoforms, and their used concentrations. Due to these differences, the results are rather difficult to compare. Various studies classify Mo related nanomaterials as very toxic, mildly toxic or non-toxic. Similarly, the mechanisms of toxicity proposed in some studies are different, including oxidative stress induction, physical membrane disruption or DNA damage. Quite promising, however, are the potential medical applications of MoS₂ nanoparticles in therapy of cancer and Alzheimer’s disease. Further studies on biocompatibility of nanomaterials based on Mo compounds are warranted. Int J Occup Med Environ Health. 2020;33(1):1–19