Opioids can cause extra pain - huh

I didn't know this.  From the start of an article at NPR:

When patients arrive in the emergency room, nearly all but those with the most minor complaints get an IV.

To draw blood, give medications or administer fluids, the IV is the way doctors and nurses gain access to the body. Putting one in is quick and simple, and it's no more painful than a mild bee sting.

Yet for some patients, this routine procedure becomes excruciating. On my shifts as an emergency physician, I began to notice a strange pattern. These hypersensitive patients often had a history of using opioids.

Shouldn't these patients be less susceptible to pain, instead of more so?

As I looked into it, I found that I was far from the first to notice the paradox of heightened pain sensitivity with opioid use. An English physician in 1870 reported on morphine's tendency to "encourage the very pain it pretends to relieve." In 1880, a German doctor named Rossbach described a similar hypersensitivity to pain with opioid dependence.

A century passed before the phenomenon received serious scientific attention. 

Turns out it is still not well understood.  Read the whole thing.