"These findings reveal that the higher-dose consumption of medical cannabis components (phytocannabinoids and terpenes) is associated with ADHD medication reduction. In addition, high dosage of CBN was associated with a lower adult ADHD self-report (ASRS) score."
As with much of the cannabinoid research these days, this study from late 2020 comes from Israel and was published in the Rambam Maimonides Medical Journal. One interesting part of the study demonstrated that higher doses of CBN specifically was related to lower ADHD symptom frequency.
The complete summary is here.