Not OP, but this isn’t always true. Might be true for slow brewing depression, but not for biologically triggered mental changes (triggered by some diseases, some drugs, etc). There’s also the chance that it was a gradual uptick that wasn’t noticed until it was too late.
Sorry for your loss OP.
I'm not sure. Drugs and alcohol related things remains drugs and alcohol related. It's not about depression, but altered consciousness. Even diseases doesn't came immidiately, they slowly developing in years, or at least months.