The SMON Incident — Birth of Japan's Post-Marketing Surveillance— An intestinal disinfectant called clioquinol caused permanent nerve damage in approximately 11,000 people over fifteen years

01Why this case matters

If the Thalidomide disaster (covered in Drug Disasters 02) exposed the failure of pre-approval safety testing, SMON exposed a different and equally fundamental gap: the absence of systematic monitoring after approval. Clioquinol was a legitimate, properly approved drug. There was no formulation defect, no fraudulent labeling. Yet a severe neurological side effect went undetected in Japan for fifteen years — during which time the drug remained on every pharmacy shelf, prescribed daily by physicians who had no reason to suspect it.

Three questions run through this case. Why did a drug already flagged as dangerous abroad remain in unrestricted use in Japan? Why did it take fourteen years to identify the cause despite a steadily climbing patient count? And who bore legal responsibility — the manufacturers, the prescribing physicians, or the regulatory authority? The answers Japan eventually gave to those questions produced the adverse-event reporting, re-examination, and re-evaluation systems that form the backbone of the current Pharmaceuticals and Medical Devices Act (薬機法). Anyone working in the Japanese pharmaceutical industry today works inside a regulatory architecture built on this case.

02Clioquinol — what the drug was

Clioquinol (chemical name: 5-chloro-7-iodo-8-hydroxyquinoline) was originally developed as a topical antiseptic for skin infections. By the 1930s it was in use across Europe and North America as an external treatment for fungal skin conditions — a role well within its established safety profile.

In postwar Japan, however, the drug migrated to a very different application: oral intestinal disinfection. Its antibacterial properties made it attractive for treating travelers' diarrhea and chronic enteritis, and multiple manufacturers produced and sold clioquinol-containing antidiarrheal tablets under various brand names. Physicians trusted the products; prescribing thresholds were low. The tablets also circulated as over-the-counter preparations, meaning patients sometimes continued them on their own initiative well beyond any prescription period.

Warning signs from abroad went unheeded. In Sweden, scattered reports of neurological symptoms linked to oral clioquinol led authorities to ban its internal use in 1966. No equivalent mechanism existed in Japan to receive that signal, evaluate it, and translate it into regulatory action. The drug sold on.

031955–1969 — fourteen years of accumulating harm

SMON stands for Subacute Myelo-Optico-Neuropathy — a term coined by the neurologist M. Kono in 1964 to describe a syndrome being seen with growing frequency in Japanese hospitals. The clinical picture was consistent: sensory disturbance and numbness in the legs, followed by progressive gait difficulty and lower-limb paralysis, and in severe cases visual impairment or complete blindness. In most patients the deficits were permanent.

Cases were first noted sporadically around 1955 and accelerated sharply through the late 1960s. The patient population was disproportionately adult women with a history of chronic intestinal complaints — a detail that in retrospect pointed directly at the drug, but was not read that way at the time. The dominant hypothesis in Japanese neurology and internal medicine was viral disease. Some hospitals isolated SMON patients out of fear of contagion, channeling research resources into tracing infection routes rather than examining shared medication histories.

The early symptoms of SMON — leg numbness, sensory loss — are not distinctive on their own. They overlap with dozens of common conditions: diabetic neuropathy, vitamin deficiency, disc disease. Each physician seeing one patient in one clinic had no way of knowing that thousands of others across the country were presenting with the same picture after taking the same drug. There was no mechanism to aggregate individual clinical observations into a national pattern — and that structural absence was the most important reason fifteen years passed before anyone connected the dots.

041970 — Professor Tsubaki's discovery

The pivot came in 1970. Professor Tadao Tsubaki, a neurologist at Niigata University, approached SMON as an epidemiologist rather than as a clinician hunting for a pathogen. He mapped the geographic and temporal clustering of cases, then systematically examined what patients had in common beyond their symptoms.

What emerged was unambiguous: the overwhelming majority of SMON patients had a history of prolonged oral clioquinol use. Hospital cluster outbreaks, which had been interpreted as evidence of contagion, corresponded instead to wards where the drug was commonly prescribed. There was also a striking clinical detail — some SMON patients' urine and stool had a greenish discoloration, consistent with the chelate compounds clioquinol forms in the body.

"The cause of SMON is clioquinol. The epidemiological evidence is already sufficient." — Professor Tadao Tsubaki, report to the Ministry of Health and Welfare, 1970

Tsubaki reported his findings to Japan's Ministry of Health and Welfare in August 1970. At that point, the precise biological mechanism by which clioquinol damaged the spinal cord and optic nerves had not been fully worked out in animal models. But the epidemiological evidence was solid enough to act on. Epidemiology can point to a cause before the mechanism is understood — and this case remains one of the clearest demonstrations of that principle in twentieth-century drug safety history.

05September 1970 — suspension, and the proof that followed

Acting on Tsubaki's report, Japan's Ministry of Health and Welfare issued a suspension of manufacture and sale of all clioquinol-containing preparations in September 1970. For a Japanese regulatory authority of that era, the speed of response was exceptional.

What happened next provided the strongest possible confirmation that the attribution was correct. Within weeks of the suspension, the flow of new SMON cases stopped — almost completely. Reporting that had been running at dozens of new cases per month converged to near-zero by October 1970. In epidemiological terms this is a near-perfect natural experiment: remove the exposure, the disease stops. The intervention itself became the proof of causation.

But one question demands the same attention as the suspension's speed: why was the suspension only September 1970? Sweden had banned internal clioquinol use in 1966. Domestic case counts had been rising visibly since the mid-1960s. The swiftness of the government's eventual response is to its credit; the duration of its prior inaction is equally part of the record. Both facts belong in any honest account of this case.

06Litigation and settlement — the three-party responsibility doctrine (1971–1979)

SMON victims began filing lawsuits in 1971. Cases were brought in district courts across the country, ultimately drawing more than 6,000 plaintiffs. The defendants named were the pharmaceutical manufacturers, the national government (Ministry of Health and Welfare), and the prescribing physicians — a configuration that had no precedent in Japanese drug liability law.

Settlements were reached at courts across Japan in 1979 (the "SMON litigation settlements"). Manufacturers and the government jointly compensated victims through lump-sum payments and ongoing medical expense coverage.

The most significant legal principle established by the litigation was the concept of joint and several liability of government, industry, and physicians in drug-harm cases. Japanese law had not previously made it easy to hold the regulatory authority itself legally responsible for its inaction. The SMON settlements established, as a recognized precedent, that a failure to regulate — a failure to act — could itself constitute actionable legal responsibility. That principle has shaped Japanese pharmaceutical administration ever since.

07The 1979 Pharmaceutical Affairs Law amendment — building the PMS system

The legislative response to SMON was the 1979 amendment to Japan's Pharmaceutical Affairs Law (薬事法). The institutions it created are not historical artifacts: they are the direct ancestors of the post-marketing safety architecture in force today under the Pharmaceuticals and Medical Devices Act.

These systems answered directly the structural failure SMON exposed: no one was systematically watching approved drugs. The 1979 amendment encoded a new principle into Japanese pharmaceutical law — approval is the beginning of surveillance, not the end of it.

08Four lessons that still hold

Lesson 1 — Foreign warnings that never arrived

Sweden banned oral clioquinol in 1966. Japan's suspension came four years later. In those four years, the Swedish regulatory action generated no binding response in Japan — not because the information was hidden, but because no mechanism existed to receive it, evaluate it, and convert it into domestic action. SMON is the foundational case behind Japan's participation in international pharmacovigilance frameworks: the ICH guidelines, the CIOMS working groups, the WHO-UMC global adverse-event database. The question SMON posed — how does a safety signal in one country reach regulators in another quickly enough to matter — is still not fully answered.

Lesson 2 — The "familiar symptom" misdiagnosis trap

Leg numbness and sensory loss are the kind of symptoms internal medicine and neurology see every day. They overlap with diabetic neuropathy, B12 deficiency, lumbar disc disease, and dozens of other conditions. A drug-induced cause will not stand out from within a single clinic. What aggregated adverse-event surveillance does is reveal patterns invisible to any individual clinician: the fact that leg numbness is suddenly increasing, that it clusters among patients with enteric complaints, that it correlates with a specific prescription. Individual medical excellence cannot substitute for that system-level view.

Lesson 3 — Regulatory inaction is itself a form of action

SMON litigation established that the Ministry's failure to act — its failure to seek out overseas safety data, to build surveillance capacity, to issue precautionary guidance — was not a neutral absence but a legally cognizable failure. Pharmaceutical regulators do not discharge their duty simply by reviewing new drug applications. They bear an ongoing affirmative obligation to monitor the post-approval environment, evaluate emerging signals, and move when the evidence warrants it. This principle is now embedded in the mandate of Japan's Pharmaceuticals and Medical Devices Agency (PMDA), founded in 2004.

Lesson 4 — Approval as a conditional permission, not a final verdict

Before SMON, a drug approved in Japan was effectively approved permanently, absent some dramatic safety event. Approval functioned as a permanent licence. The re-examination and re-evaluation systems introduced in 1979 formally rejected that model. Approval became a conditional and periodically revisable determination — one that depends on real-world data that cannot exist at the time of the original application. This conceptual shift, that a drug must continue to earn its place on the market through ongoing demonstration of an acceptable benefit-risk balance, is the intellectual foundation of modern pharmacovigilance globally, not only in Japan.

09Connections to other chapters

• Drug Disasters 02 — Thalidomide — Thalidomide institutionalized pre-approval safety testing; SMON institutionalized post-approval surveillance. The two cases together built the two pillars of Japan's modern pharmaceutical regulatory system
• Compliance Vol. 1 — The principle that manufacturers, government, and physicians share joint responsibility for patient safety — established in the SMON settlements — is the legal articulation of what it means to operate in a life-related industry
• Ad Regulations Vol. 1 — The post-SMON strengthening of package insert requirements and adverse-event disclosure obligations flows directly into the labeling duties governed by the Pharmaceutical Act today
• Material Review series — The obligation to accurately reflect post-market safety information in promotional materials traces its regulatory lineage through this adverse-event reporting system