What would affect how long a pills effects- lasts?

A drug (medicine) I used , used to last 72 hours, now all of a sudden it lasts me only about half of that?

What changed in my body

?2017-03-27T21:21:50Z

its called tolerance. your body is getting used to its effects and is building up a tolerance to it.
over time, sometimes only weeks! needing higher doses for the same effect is common. especially in pain meds!!
your brain changes.
either that or your condition / injury is getting worse.