Q. Both CMOS 10.27 and Merriam-Webster style “post-traumatic stress disorder” that way, with a hyphen, but it occurs to me that since PTSD takes hold in the wake of traumatic stress, the prefix post- applies to “traumatic stress.” So shouldn’t the spelled-out term be styled “post–traumatic stress disorder,” with an en dash?
A. You’re right: It would make sense to spell post-traumatic stress disorder with an en dash. In fact, we could have used your editorial wisdom back in 2003, when we added posttraumatic to the fifteenth edition of CMOS to show that the prefix post- forms one-word compounds even if the result is two consecutive t’s.
That decision would have reflected the spelling posttraumatic in a list of 125 compounds formed with post- under the entry for that prefix in the then-current tenth edition of Merriam-Webster’s Collegiate Dictionary.
Still, we must have failed to register the hyphen in Merriam-Webster’s separate entry for post-traumatic stress disorder, which followed entries for posttranscriptional, posttransfusion, and posttranslational—all without hyphens. It’s clear in hindsight that, as you suggest, post-traumatic was hyphenated in that term (and not in the list of words formed with post-) for a reason, one that had nothing to do with consecutive t’s. (The latest iteration of Merriam-Webster, as of November 4, 2025, now hyphenates post-traumatic as a standalone term also.)
But the dictionary doesn’t do en dashes,* and in post-traumatic stress disorder, no one else seems to either (not even Wikipedia, which tends to feature a lot of en dashes compared with other publications). Instead, we’d advise saving your en dashes for more established uses (see CMOS 6.82–90—especially 6.86, on en dashes with compound adjectives).
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* The hyphenated main entries in printed editions of Merriam-Webster do feature en dashes rather than hyphens, but that’s presumably for the sake of legibility in very small type and therefore doesn’t count (see also this related Q&A).