If you're going to get hospitalized this summer, try not to do it in July. That appears to be the month when fatal medication errors in hospitals spike, new research suggests.

At least, that's what researchers found for patients treated in areas with teaching hospitals -- those hospitals in major centres where new medical school graduates start to learn about work on the front lines.

Researchers are calling the spike "the July effect" and they say it looks like it's linked to new, inexperienced doctors entering residency programs at hospitals.

According to the recent study published in the Journal of Internal Medicine, there is typically a 10 per cent spike in fatal medication errors in July, but not in any other month.

Sociologists at the University of Calfornia San Diego came to their conclusions after studying more than 62 million U.S. death certificates from between 1979 to 2006. They focused on 244,388 deaths linked to medication errors.

The death certificates didn't record whether the patients died in a teaching hospital, but they did record the county of death. So the researchers looked at the proportion of major teaching hospitals in each county.

They found an average increase of 10 per cent in medication-linked deaths in July in counties with teaching hospitals, but none in other counties. In fact: "the greater the concentration of teaching hospitals in a region, the greater the July Effect for intra-institutional medication errors in that region," they write.

The researchers say they tried to test other explanations for the deaths, other than that new residents were to blame.

For example, if the July Effect had come from greater alcohol consumption in the summer, or other unique summertime behaviour, then the spike also would have been seen in August too. But it wasn't.

Ultimately, the arrival of the new medical residents was found to be the most likely cause of the increased fatal medication errors in July.

"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," they write.

It's possible that new residents make more mistakes because they are sleep-deprived and still learning the system.

Whatever the reason, the researchers say residents should be watched carefully. Their findings, they write, "provide fresh evidence for

1) re-evaluating responsibilities assigned to new residents;

2) increasing supervision of new residents; [and]

3) increasing education concerned with medication safety."

The good news is that the research suggests new residents appear to learn quickly: within a month, medication errors fell back to normal rates.