In the face of a series of accidents caused by drivers high on law-skirting "dappo" drugs, the health ministry used new emergency powers Tuesday to ban two substances.
The substances are believed to have been used by a man who allegedly killed one pedestrian and injured seven while driving under the influence of the drugs in the Ikebukuro district of Tokyo on June 24.
Keiji Nagura, 37, reportedly told police he "inhaled dappo herbs."
The police said the "herbs" he smoked included two substances that have the same effect as cannabis.
Also Tuesday, police in Nagoya arrested Takehiko Nishikawa, a 49-year-old company executive, on suspicion of driving in January under the influence of dappo drugs.
The substances he allegedly used were recently added to the list of banned chemicals, police said.
The health ministry generally spends some six months designating substances that likely have stimulant or hallucinatory effects and that are considered harmful. Substances labeled as such cannot be sold or possessed.
Following the Ikebukuro accident, however, the ministry decided to resort to an emergency measure to designate the two substances as harmful without undergoing the usual procedure of checking their toxicity and listening to the views of experts.
It is the first time that the ministry has used the emergency process to address loophole drugs, which are not technically illegal but which induce effects similar to illegal narcotics.
It only took about three weeks for the designation of the drugs in question and the change will take effect July 25.