In a multi-decade career I only actually observed drug use (Marihuna) once (difficult situation in the company and the fellow just didn't care anymore); not counting alcohol during festivities (neither counting coffee or nicotine). I suspected it multiple times though. The boss with the wide-eye super-intense stare and unnatural, not to say, crazy, enthusiasm -- was that cocaine? The co-worker first giddy, later unbearably mean -- was that bipolar disorder or waning anti-depressant? Who knows.