Dr. Tom recently flagged this study, which found that among older adults, regular Xanax use was correlated with increased likelihood of developing Alzheimer's. Here's a summary in the Harvard Health Blog, and a nice brief analysis in the Washington Post.
When I think of Xanax, the primary two risks I think about are high potential for dependence, and dangers of mixing it with other drugs (especially alcohol). I hadn't heard much in the way of persistent, negative long-term effects. So I was surprised to read this, and more than a little concerned, given how common Xanax use is among people with anxiety.
Of course, there are caveats (thankfully). This is only one study, and it employed a correlational methodology that can't prove that Xanax causes Alzheimer's (though they did make some effort to correct for this). And, the study population is comprised entirely of older adults. So it's unclear whether Xanax use in younger people carries the same (or any other) risk.
But, caveats aside, studies like this provide a healthy reminder that we never fully can predict the long-term impact that a medication might have.