"I wonder if there is any correlation between when the OD's occur and the time of the month when the welfare checks arrive."
My thoughts exactly.
The story did not make it clear if the data included overdoses related to common consumer errors such as taking medications while too intoxicated, or being too impaired to prevent small children from helping themselves to your meds.
I have witnessed a number of such cases over the years. I have also seen a tendency for some people to enjoy alcohol or drugs a little too much when the checks come in. I wonder if this is part of what the researcher was measuring.