Jon Cryer‘s opening up about his relationship with Demi Moore back in the day … revealing he had no idea about her drug addiction while they were together.

The actor — who starred alongside Moore in the hit 1984 flick “No Small Affair” — tells People — briefly dated his costar while making the classic movie … but, JC didn’t realize she was using until about two weeks before wrap.

The way Jon explains it … their tryst ended fairly quickly, and he remembers her as having a charismatic spark to her — all of which completely fooled him.

Cryer also says Demi knew just about everyone in town … coming off as a super charming person while clearly dealing with her own set of demons.

The two reconnected last night at the premiere of “BRATS,” the documentary film about the legendary “Brat Pack” … young ’80s actors who starred in some of the most iconic films of the era like “The Breakfast Club” and “St. Elmo’s Fire.”

Moore starred in the latter film too … though not before going to rehab at the request of the film’s director Joel Schumacher. Once out, Moore said in her memoir she managed to stay sober for nearly 20 years before relapsing.

Demi was just one of many members of the Brat Pack to struggle with addiction … Robert Downey Jr., Rob Lowe, Andrew McCarthy, Ally Sheedy and many more also entered rehab.

Bottom line … lots of these stars needed addiction help — and, it seems not even those closest to them always knew. “BRATS” drops June 13 on Hulu.