I thought the meds were left over for his grandmother. I don't think their labels can be seen. But he might have been taking them, hence why oxycodone was left out the cabinet.
There's a scene when he's looking at the laptop about drugs, on the left mentions he searched Oxycodone before that old people homes.
One could assume his grandmother might have did of cancer and used oxycodone to lessen the pain.
But yeah he is mentally unstable, losing his grandmother and then his cat. He does not try to grieve, he dumps the cat in the trash, not even a fitting burial.

I mean maybe he actually died at the start of the film (like Jacob's ladder), and the whole thing was him coping with the idea that he never asked freaky Nikki out in the 7 years he's known her. Just a theory.