<p>Pharmaceutical products are approved through a long and careful process to secure the balance of safety and efficacy, however, there is a risk of unpredictable events in a real world because of limitations of settings of clinical trials. Pharmaceutical companies have a responsibility to monitor the use of approved drugs and then promote their appropriate use based on latest information after approval in parallel with stable supply. It is expected that N–of–One medicine could provide clinical benefit to a very small number of patients by minimum changes in the sequence of approved nucleic acid drugs. On the other hand, it is quite difficult to predict effects of such changes and differences of application on the balance of safety and efficacy under the condition of clinical study with a highly limited number of participants. Additionally, quality control for products of small quantity and wide variety does not seem to be equal to that for a single product of larger quantity, resulting in the difficulty of the use of a common product line. Additionally, quality control for products of small quantity and wide variety does not seem to be always equal to that for a single product of larger quantity, resulting in the difficulty of the use of a common product line. As described above, a framework to secure the balance of safety and efficacy of N–of–One medicine with a controlled quality for the benefit of corresponding patients is more important than financial challenges. Roles and responsibilities of industries in the frame, together with those of the regulatory authority and medical institutes, needs to be clarified and then its social consensus must be made.</p>