Duration of action for pain medication can differ extremely among individuals.<p>When my wife (chronic pain patient) wanted to switch from one pain med to another because of diminishing effect, I helped her neurologist review literature on the equianalgesia properties of various options.<p>For several meds, blood concentration vs. time varied by one order of magnitude among patients who were otherwise well-matched subjects. Some metabolized&#x2F;eliminated a med to non- therapeutic levels in a day, others took nearly two weeks. Same dose, same route of administration. Probably different P450 enzyme activities [1] among other things.<p>I came away from the experience appreciating how incredibly difficult it is to construct a good pain management plan. It takes more time and info than many doctors can bring to bear on each patient.<p>[1] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Cytochrome_P450" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Cytochrome_P450</a>