The "Can't Hardly Wait" star was in Larchmont Village Tuesday and the convo started on a serious note -- drug addiction. Ethan was hooked on heroin and painkillers, but says he's 6 years sober now.

He railed against those who want to criminalize drug addiction -- going off on a Twitter rant to get his point across ... but in the midst of that convo, we learned he's picked up quite the fitting habit.