Initially a news source reported that Ilhoon had been engaging in illegal marijuana usage from 4-5 years ago up until last year.
The police were able to trace the incidents to him, find electronic transactions of him purchasing the drug, and the drug was detected in a hair sample of his.
What makes this even more serious, is that it is unknown if Ilhoon was using the drug while in the military.

His company initially released this statement:
"We have no idea about Ilhoon having a drug scandal. He was initially scheduled to join the military in March, but his term was delayed due to COVID-19."

Then they later released more of a confirmatory statement:
"Hello, this is Cube Entertainment.
After checking with Jung Ilhoon in regard to the news released earlier today, it has been confirmed that he has been summoned by the police investigation agency for questioning on charges of smoking marijuana, as previously reported.
He feels a heavy responsibility for causing concern to many people, and he will do his best to faithfully carry out future investigations.
Once again, we are sorry for causing concern."

I'm not exactly sure what consequences Ilhoon could face, if he is found guilty of the offense.
Although my stance on marijuana usage is nearly opposite of Korean law, it has to be respected in this situation.