Police last year recorded 706 cases involving quasi-legal drugs, a more than five-fold increase since 2013.
The figure represented an all-time high since comparable data became available in 2008.
The surge is partly attributable to recent moves to tighten control over what officials currently refer to as kiken (dangerous) drugs, which have effects similar to illegal narcotics but were not until recently proscribed by law. They were previous called dappo (loophole) drugs for their ability to skirt the law by varying their recipes with myriad ingredients not yet outlawed.
The drugs have been implicated in traffic accidents, in which four people have died, including an elementary school student. But they also exact a toll on users: A total of 112 people are thought to have died after taking them.
The Health, Labor and Welfare initially tried to restrict production, sale and import of the quasi-legal drugs, but after these attempts failed, a revised pharmaceutical affairs law went into force in April last year banning the use and possession of such drugs.
A total of 401 cases, the largest portion, involved violations of the former and revised pharmaceutical affairs laws, and 157 related to violations of traffic-related laws.
The number of drug abusers, excluding those who were involved in importing or selling the drugs, totaled 631 people, of whom about 70 percent were in their 20s and 30s. The youngest drug abuser was aged 15.