“Imitation” very strongly implies that it is not in actuality the thing being imitated. Imitation butter is not butter. Imitation crab is not crab.
These medicines are the same chemical, therefore the same product.
I will however grant that while calling store brand painkillers “imitation ibuprofen” is nonsense, calling them “imitation advil” is okay because advil is a brand. Though in my opinion it should be avoided because it carries an implication of inferiority that is simply not the case.
Society equating depression with sadness is a great disservice to the condition. It’s quite common for it to present as just … nothing. An emotional void where you might expect emotions to be. Things that would be expected to make you happy just don’t. Things that would make you sad, the same. Your feelings are depressed in the sense that their impact is just muted across the board.
A lack of motivation is also a very common indicator. You’re just missing the drive to do something because the emotional rewards that you expect to happen when you accomplish your goals just aren’t there.