A total of 4,469 people were taken to hospitals in connection with so-called quasi-legal drugs from January 2009 to June this year, the Fire and Disaster Management Agency said Friday.
In the first survey of its kind, the agency collected reports from regional emergency services on people thought to have been under the influence of "dappo" (loophole) drugs, which the government officially terms "dangerous drugs."
The number of people taken to hospitals grew from 30 in 2009 to 85 in 2010, but spiked to 602 in 2011. It peaked the follow years at 1,785 before easing to 1,346 in 2013. The tally for the first half of this year is 621.
Tokyo reported the most cases with 1,130, followed by Osaka with 536 and Kanagawa Prefecture with 512.
The agency cautioned that the numbers can only serve as an indicative reference, because substances believed to have caused symptoms were not confirmed in each case.
The survey results come amid a crackdown on the drugs following a series of traffic accidents involving drivers who consumed them.
On June 24, a 37-year-old man under the influence of such a drug drove a car into pedestrians in the Ikebukuro district of Tokyo's Toshima Ward, killing one person and injuring six others, according to the indictment.
The Health, Labor and Welfare Ministry has listed a 1,400 substances as dangerous drugs, banning their sale or possession.