The paper, recently published in The Lancet Regional Health - Americas, analyzed almost 12 million samples of different illicit substances, from heroin & meth to marijuana. The samples were collected from 2013-2023, from across the US, to help identify any trends in fentanyl content.

While fentanyl was found quite commonly in some substances (around 50% of heroin samples), the study showed that cannabis was the least likely substance to contain fentanyl.

Here is the overall breakdown they found, directly from the paper itself:

This study was conducted by researchers at Harvard Medical School, Brown University, and the Food & Drug Administration (FDA), with federal funding. They used data from all 50 states and the District of Columbia, from the National Forensic Laboratory Information System, which is the US’s largest database of illegal drugs.

Read the whole paper:

https://www.thelancet.com/journals/lanam/article/PIIS2667-193X(24)00225-4/fulltext 

Journal Reference:

Tse Yang Lim, Huiru Dong, Erin Stringfellow, Zeynep Hasgul, Ju Park, Lukas Glos, Reza Kazemi, Mohammad S. Jalali, Temporal and spatial trends of fentanyl co-occurrence in the illicit drug supply in the United States: a serial cross-sectional analysis,

The Lancet Regional Health - Americas, Volume 39, 2024, 100898, ISSN 2667-193X, https://doi.org/10.1016/j.lana.2024.100898