Yeah, that's a good idea. I could add multiple Painkillers scattered around and each one allows you to run for a few minutes. The main character could start with an active Painkiller effect, so that when it runs out, he says something like "Ah! My back hurts! I need some Painkillers". The player would then immediately associate the slow walking animation to the Painkiller effect and look for one.
This could make up for some interesting situations!