Significant interest in the application of chloroquine (CQ) and hydroxychloroquine (HCQ) for prophylaxis and treatment of COVID-19 has been recently reported throughout the world. In the light of this, it has become important to revisit the mechanisms of action, as well as clinical evidence for the efficacy of CQ and HCQ in similar viral infections. We note that while some evidence exists for therapeutic efficacy of CQ and HCQ in mitigating morbidities associated with virus-induced inflammation and immunohyperactivity, abundant caution is necessary for expecting prophylactic efficacy in the light of therapeutic evidence. In particular, we intend to draw urgent attention to the fact that the same immunosuppresive and immunomodulatory properties of CQ and HCQ which render them useful as therapeutic agents in viral pneumonia and cytokine storm syndrome are also expected to make them dangerous prophylactic agents against viral infections. We also intend to draw attention to some other potential concerns associated with widespread prophylactic usage of CQ and HCQ.