I recently heard something amazing: July is the worst month in which to undergo a medical procedure because the risk of a mistake is higher in July than in any other month of the year. But why?

What’s so special about July? If you don’t already know the answer, I recommend you pause reading this article now and see if you can come up with the answer.

My first thought was that in July many doctors are on vacation or are mentally distracted by summer vacation plans. That explanation is a bit like the old adage about avoiding a car that was manufactured on a Monday or a Friday (due to back-to-work doldrums and weekend anticipation, respectively). That answer turned out to be wrong. If you’re still pondering, I’ll give you a big hint: it’s related to a certain fixed timetable.

The answer is that July is the month when a brand new crop of interns and residents are unleashed upon the general public. Every July the system is flooded with rookies. And rookies make rookie mistakes. But this sounds like an urban myth, doesn’t it?

I found a study which corroborates this claim, finding a 10% spike in deaths due to medication errors in July, primarily in US counties with teaching hospitals. The study concludes thusly:

We found a significant July spike in fatal medication errors inside medical institutions. After assessing competing explanations, we concluded that the July mortality spike results at least partly from changes associated with the arrival of new medical residents.

There’s really never a good time to be in the hospital, but if you believe this study, July is a particularly bad time of year to undergo significant medical treatment, especially at a teaching hospital.