As director-general of the Pharmaceutical Safety and Environmental Health Bureau of Japan’s Ministry of Health, Labour and Welfare, I cannot accept your criticism of our approval of stem-cell treatment for spinal-cord injuries (see Nature 565, 535–536; 2019 and Nature 565, 544–545; 2019).

Your criticism is based on the absence of double-blind studies for this treatment. But in this therapy, known as Stemirac, stem cells from the patient’s bone marrow are cultured externally and then returned to the patient. A double-blind study is therefore structurally impossible, and performing a sham operation on a control group would raise ethical issues.

In such cases, properly designed clinical studies can still test efficacy — as demonstrated for drugs approved by the US Food and Drug Administration as well as in Japan. Given the convincing response to Stemirac by the group of paralysed people under discussion, it could be unethical to withhold approval and deny treatment. The rationale for the safety, efficacy and quality of the product, and for the ethics of its approval, is given in the evaluation report by Japan’s Pharmaceuticals and Medical Devices Agency (see Japanese and English versions).

You also criticize Japan for marketing products with ‘questionable’ efficacy and for making patients bear the costs of clinical studies. However, under the terms of the country’s conditional and time-limited approval for regenerative medical products, such products are granted marketing authorization only when efficacy can be demonstrated in post-marketing studies within a specified period. And, because Stemirac is covered by national health insurance, patient payments are fixed at a feasible level.