Its a rather peculiar feeling, the best way to describe it is like going under anaesthesia.
From the first prick of the needle finding its way into your vein. It hurts, but you slowly get used to the pain.
You know that the poison running through your veins is pulling you away from reality and into the fog.
Your eyes become cloudy. The once sharp, vivid, clear voices become hazy and distant.
The poison is setting in. You try to stay but there is nothing keeping you from leaving.

Finally you let go.

The soft sedate hands of the poison drag you under.

All goes numb, and the dark becomes the only thing visible.