One of the most promising strategies for drug design and development is the identification of new molecules able to selectively inhibit those enzymes involved in pathological processes, without affecting other enzymes associated with physiological functions. Nevertheless, some enzymes can show a double-edge aspect of their own inhibition, which can lead to positive as well as negative consequences according to the pathological state. Phospholipase D (PLD), an ubiquitous enzyme nowadays considered as a critical regulator of several aspects of cell biology and signal transduction pathways, is a clear example of those double-dealer enzymes. While a great deal has been learned about PLD structure, biological functions and activation/regulation mechanisms, little yet is known about the derivable effects of its potential negative regulation, also due to the lack of specific inhibitors. Multiple evidences on PLD involvement in many pathological states development and progression, including inflammation, carcinogenesis and metastases, have been supplied, so that a deregulation of its activity could contribute to attenuate or slow down the inflammatory and tumour formation/ progression processes. On the other hand, in agreement with other previous observations, we have recently demonstrated the direct contribution of PLD activation in promoting intracellular mycobacterial killing. In this case, PLD inhibition resulted in a significant reduction of antimicrobial innate immune response and, hence, in a possible harmful effect. In the light of the above reported considerations, besides the recent advances in characterising new compounds able to selectively inhibit PLD activity and/or signalling, this review aimed at elucidating the potential dual beneficial/harmful consequences of PLD activity modulation.