And one more ranty thing about Dexter (just ignore me):
If you want to kill someone, there has to be no less efficient plan than what Hannah apparently did to Deb. i.e. - dissolving some extra Xanax in her water. I speak from extensive experience with benzodiazepines.
Let's set aside the change in appearance and taste of the water which Deb would notice big-time - but okay, forget that.
The chances that someone will actually be suddenly hit like a ton of bricks by the dose and pass out cold at the exact moment they are driving at a lethal speed must be about .0001%
It doesn't work like that. It is not a knockout drop experience. What happens is, you gradually get tired. Even if Deb absolutely chugged her whole water - which is far from something the killer could feel is guaranteed - it is still a gradual onset. There is a period of not feeling right.
So first of all, our master killer doesn't even know if Deb will be driving when the dose starts to set in; she could be doing anything. But let's say she is at the wheel of a car. She would start to feel off. Any non-moron would slow down, pull over, possibly call someone. There is plenty of time.
And then, depending on your resistance, you would drift off to sleep. Not necessarily. You can fight it by will. And if it is a drug you are already taking - which was the case here - your body may have adapted to the sedative effect.
Basically all that Hannah did was make it probable that Deb would have a nice nap at some indeterminate future time.
Okay end of ranting.
Maybe.