Research on cannabinoids has been growing significantly in the last five years. More than fifty percent of this research corresponds to “cannabinoids and brain”, particularly about neurodegeneration. In this sense, there is evidence reporting that specific phyto cannabinoids show some specific action on each one of main pathogenic mechanisms involved in neurodegeneration such as oxidative stress, neuroinflammation and excitotoxicity. However, by using the same targets, cannabinoids may also induce the opposite effects, this is, excitotoxicity and inflammation. In fact, both tetrahydro cannabinol and cannabidiol activate cannabinoid receptors, but they also may act as antagonists of those receptors. It seems to be a dose-dependent issue; nonetheless, as reviewed in this paper, many other factors such as timing, type of cell and its state of activity even the activation of different, non-cannabinoid receptorsseem to have a role related to those unexpected antagonic effect.