When and how drugged?

*** Spoilers, obviously ***

When and how was he drugged? Did I miss something?
After he is thrown into the river from the bridge we see him shivering in the police station. Although he could just have been cold from the water, it does appear more than that, and that he's already disoriented from the early stages of the 'coma' drug.
Presumably administered at the police station or after being fished out of the river.

And why a gun in the fridge with Barbara? He didn't need it as a framing 'murder weapon' as his friend was stabbed, not shot.

I don't need to be spoon-fed everything, but it's hard to know whether story telling elements have been omitted on purpose, or due to filming/edit/budget reasons.

Perhaps all these little mysteries help create a bit of disorientation in me, the viewer, and contribute to the film's quite disturbing feeling.