Japan Considers Six-Sanatorium Inquiry Into Wartime Koha Drug Trials

180 Million Yen Set Aside to Examine Drug Use

Japan's Ministry of Health, Labour and Welfare is considering an investigation at six national leprosy sanatoriums into the use of Koha, an experimental drug developed under the former Japanese Army and administered to residents during and after World War II. The government has secured 180 million yen in its fiscal 2026 budget for the work.

The proposed inquiry would expand scrutiny beyond Kikuchi-Keifuen in Kumamoto Prefecture, where an investigation found that at least 472 residents, including children as young as six, received the drug between December 1942 and 1947. Nine residents died during those trials, and investigators suspected that Koha caused two of the deaths.

A ministry official disclosed the plans on November 9, with the proposed investigation reported on November 10, 2026. The six institutions and the starting date have not been decided. Investigators would examine records held at each institution once preparations are complete, excluding Kikuchi-Keifuen because it has already conducted its own inquiry.

The central issues are how widely the drug was used, what happened to the people given it, and why treatment continued despite serious illness and deaths. The earlier investigation also examined whether residents, confined to institutions and dependent on their doctors, had any meaningful ability to refuse.

Evidence Extends Beyond One Institution

Records have already confirmed Koha use at five national sanatoriums besides Kikuchi-Keifuen: National Suruga in Shizuoka Prefecture, Oshima-Seishoen in Takamatsu, Tama-Zenshoen in Tokyo, Nagashima-Aiseien in Okayama Prefecture and Hoshizuka-Keiaien in Kagoshima Prefecture.

Those findings emerged from inquiries to the 12 national sanatoriums other than Kikuchi-Keifuen. Evidence appeared in medical records, an institutional journal, a residents' association magazine and a medical journal published in 1947. Together, they show that the drug's use reached several regions of Japan.

The five facilities already identified give the proposed investigation a starting point. Its eventual scope will depend on which six institutions the ministry selects and how much documentation survives at each location.

How an Army Research Project Reached Residents

Koha was composed of cryptocyanin, a photosensitive dye, meaning a substance that reacts to light. It came from research involving photographic sensitizing agents. The former Imperial Japanese Army became interested in whether the drug could treat frostbite and burns during military operations in cold regions.

The army commissioned Kumamoto Medical University, now the School of Medicine at Kumamoto University, to research its possible uses. After apparent success in patients with tuberculosis, researchers considered testing it against leprosy, also called Hansen's disease. The bacteria responsible for the two diseases are related, which helped explain the researchers' interest.

At Kikuchi-Keifuen, then director Matsuki Miyazaki joined the research and began administering Koha to residents in December 1942. The work was commissioned by the 7th Research Center of the Army Technical Headquarters. Initially, the experiments involved about a third of the sanatorium's residents.

The treatment continued beyond Japan's surrender in 1945. At Kikuchi-Keifuen, trials lasted until 1947, while records at Oshima-Seishoen describe administration from 1944 to 1946. The proposed inquiry therefore concerns both wartime military research and medical decisions made after the war.

What the Kikuchi-Keifuen Inquiry Found

Kikuchi-Keifuen released its investigative report on June 24, 2024, after its historical archives began sorting related materials in 2022 and the residents' association requested a detailed examination. Investigators identified at least 472 people who received Koha.

Doctors used several methods: powders and tablets, injections into muscles or veins, delivery through a catheter into the bladder, and application to the membrane covering part of the eye. Single doses ranged from 0.1 to 120 milligrams, with intervals varying from daily to weekly.

Residents experienced fatigue, nausea, rashes and paresthesia, an abnormal sensation such as tingling or numbness. The report questioned the medical basis for the experiments and found that residents received insufficient explanations when their health deteriorated. It also found that appropriate treatment or compensation for side effects was lacking.

The two deaths suspected to be linked to Koha involved men aged 37 and 29. The older man developed dizziness, worsening headaches and vomiting after injections and died 34 days after treatment began. The younger man's treatment was stopped after about four and a half months because of severe lethargy; he died roughly two weeks later.

A Resident Remembers Pressure to Swallow Tablets

Tsuneji Matsumoto, an Oshima-Seishoen resident described as 93 when he recounted his experience, entered the institution in July 1942. He recalled that around early 1944 a doctor told him and other young residents that they would receive a new medicine for leprosy.

Matsumoto remembered taking a thin, flat tablet once a day before noon. Afterward, he felt dazed and his eyesight deteriorated. He recalled crawling along corridors, saying that he could not see, before being hospitalized. Other residents suffered fevers or wounds that festered, and one woman lost her hair.

According to Matsumoto, a doctor responded to complaints by presenting the adverse reactions as a sign that treatment was succeeding:

It's working because you're having a reaction,

Some residents hid tablets under their tongues and spat them out later. Matsumoto said nurses eventually began checking that each tablet had been swallowed. He remembered the treatment lasting between six months and a year, then stopping abruptly without an explanation.

His account describes how difficult refusal could be in daily life: residents who tried to avoid taking the medicine faced closer supervision.

Why the Inquiry Raised Human Rights Concerns

Kikuchi-Keifuen's report concluded that residents' human rights were not respected. They were unable to live outside the institution and were cut off from the wider community. The doctors directing the experiments also held authority over the place where residents lived.

That arrangement made it difficult to reject treatment or speak openly about its effects. Meaningful consent requires a person to understand what is proposed and be able to say no. The report described residents whose dependence on the institution sharply limited that choice.

Residents began resisting the trials in 1943, yet the experiments continued until 1947. One man, aged 97 when his recollection appeared in a sanatorium journal cited by the inquiry, described severe stomach pain after starting the drug and being called to the director's office each day to take pills under observation.

Shinzo Izumi, a doctor who has worked at a leprosy sanatorium, described the findings in stark terms:

It was a serious violation of human rights.

The inquiry said its purpose was to learn from history and improve present practice, rather than hold the people involved legally liable.

Older Records Show Frequent Treatment Interruptions

A 1947 issue of a dermatology and venereology journal, preserved at the National Hansen's Disease Museum, contains accounts of Koha use at Tama-Zenshoen and Oshima-Seishoen. These contemporary records add numerical detail to the later institutional investigations and residents' memories.

At Tama-Zenshoen, the director reported 175 cases. Treatment stopped within three months in 72 cases, while 103 continued for four to eight months. In 57 cases, administration had to stop because the resident's general health deteriorated.

Those 57 cases amount to about 33 percent of the 175 cases described, or roughly one in three. The figure gives a concrete measure of how often worsening health interrupted treatment at that institution.

At Oshima-Seishoen, the director reported administering Koha to 180 people between 1944 and 1946 and acknowledged numerous side effects that diminished after the drug was stopped. Records at Nagashima-Aiseien and Hoshizuka-Keiaien provide further evidence of use, although equivalent patient totals were not given.

Read together, the records describe a wider pattern: administration across several institutions, repeated adverse reactions and decisions about whether to continue or stop treatment.

Preserving Records and Hearing Aging Residents

National Suruga is reviewing the medical records of all 998 residents covered by its examination to determine how Koha was used. Director Shinichi Kitajima said evidence had been found in records from after the war. Staff bound by confidentiality were checking the files individually.

Kitajima described both the scale of the work and the need to protect the historical record:

There is a possibility that more unknown facts will come to light. We need to establish rules for preserving medical records and documents,

Other facilities have also faced large volumes of records and have been considering how to organize their research. Medical files can help establish who received the drug, when doses were given, what symptoms followed and why treatment ended. Death certificates are another avenue residents' representatives have asked investigators to examine.

The Health Ministry counted 639 residents in national sanatoriums in May 2025, with an average age of 88.8. Those figures give urgency to recording personal recollections alongside the documentary work. Residents can describe pressure, pain and conversations that may never have entered a medical file.

From Wartime Trials to a Proposed Wider Inquiry

The history now under examination stretches from military research during World War II to the government's proposed investigation more than eight decades later. Several dates mark how the issue developed:

• December 1942: Koha trials began at Kikuchi-Keifuen under director Matsuki Miyazaki.
• 1943: Residents at Kikuchi-Keifuen began resisting the trials.
• 1944 to 1946: Oshima-Seishoen records describe Koha administration to 180 people.
• 1947: Trials at Kikuchi-Keifuen ended; a medical journal published accounts from Tama-Zenshoen and Oshima-Seishoen.
• 2022: Work on materials at Kikuchi-Keifuen's historical archives renewed attention to the experiments.
• June 24, 2024: Kikuchi-Keifuen released its investigative report.
• November 2026: The ministry's consideration of an inquiry at six other sanatoriums became public.

Akira Ota, a representative of Kikuchi-Keifuen's residents' association, has called for investigations across the country and further examination of medical records and death certificates. He has also asked for findings to be made public and preserved for future generations.

The proposed national work would connect findings now scattered across individual institutions. For residents and families, its most concrete task is to establish a fuller account of who received Koha, the harm they experienced and the decisions made by those responsible for their care.

Key Points

• Japan has secured 180 million yen in its fiscal 2026 budget for a proposed Koha investigation at six national leprosy sanatoriums.
• Kikuchi-Keifuen's earlier inquiry identified at least 472 recipients, including children as young as six.
• Nine residents died during those trials; investigators suspected that Koha caused two deaths.
• Records confirm administration at five other national sanatoriums, with evidence of serious side effects and treatment interruptions.
• The earlier inquiry found that confinement and dependence on doctors left residents with little ability to refuse.
• Preserving medical records and recording the experiences of aging residents are central to establishing what happened.