Police dealt with 51 criminal cases linked to quasi-legal drugs in the first half of the year, according to official data from the National Police Agency.
The number of cases is up 3.2-fold compared with the first half of 2012, reflecting tighter regulation of the drugs, the NPA reported Thursday.
The number of people charged, 66, rose 2.6-fold.
Quasi-legal drugs are basically the same as illegal narcotics but contain only marginally different ingredients to evade regulation. The government has been increasing the number of such drugs that can be regulated to clamp down on them.
Of the 51 cases, 31 involved drugs found to be illicit via police investigations, up from 10 the year before.
There were 20 cases of traffic accidents, assaults and intimidation involving people under the influence of quasi-drugs. A total of 28 people were injured, the NPA said.
The number of stores selling quasi-legal drugs stood at 226 across Japan at the end of July, down substantially from 382 at the end of June last year. The number of online stores, however, climbed to 19 from nine at the end of 2012.
While tighter regulations have caused many stores to stop selling quasi-legal drugs, some may have gone underground, the NPA said.