The point as I understand/was taught it in school is that the time it would take you to pop out enough* e.g. paracetamol to do really serious lasting harm (vs. 'just' being quite sick or needing stomach pumped) gives a sort of 'cool-down' or re-evaluation period, or requires significant planning, whereas a bottle can be downed impulsively.

(* and that if you're taking them as you go, you'll probably be physically incapable/less inclined to continue far enough after a certain point)