The number of distributors exploiting loopholes to legally sell products similar to illegal narcotics soared to 389 across 29 prefectures as of March 31, a jump of more than 80 percent since mid-January, according to a recent health ministry survey.
Though similar to illegal drugs, the products' chemical composition is slightly altered to enable distributors to skirt current laws and sell them both over the counter and online.
But consuming the products, which are marketed as herbs and incense, can induce physical health problems and altered mental states, and the trade's rapid spread has forced the Health, Labor and Welfare Ministry to look at ways to tighten current regulations.
The ministry's survey showed that Tokyo accounted for the largest number of traders, at 94, or about 24 percent of the total, followed by Osaka Prefecture at 73, nearly 19 percent. Aichi Prefecture, with 34 distributors, and Okinawa with 27 posted the next highest tallies.
The trade's growth is thought due to its rapid expansion in smaller cities, with 12 of the 30 prefectures that recorded no distributors in the January poll reporting a combined 79 in the latest survey.
The earlier tally found only 212 vendors nationwide as of Jan. 18.
The health ministry is considering setting up a system to regulate such products more stringently, while the National Police Agency is also closely monitoring developments.
"We are striving to grasp a precise picture of the present situation so we can properly regulate the sector in the future," one ministry official said.