Examining the relationship between cannabis use and drinking levels on co-use days.

Background: Co-use of alcohol and cannabis among young adults is often associated with more alcohol consumption than alcohol-only use, yet little work has examined cannabis use specifically during heavy drinking. Much of the work examining heavy drinking has focused on heavy episodic drinking (HED, 4+/5+ drinks/occasion for females/males); however, young adults report drinking levels that far exceed this, frequently reporting high-intensity drinking (HID; 8+/10+ drinks/occasion for females/males), double the HED threshold, which confers greater risk of acute negative consequences. This study examined whether day-level co-use (vs. alcohol-only use) was associated with greater odds of heavier drinking levels: HID vs. HED vs. moderate drinking (1-3/1-4 drinks/occasion for females/males). We explored whether within-person variations in cannabis use characteristics (frequency, form, and quantity) differentiated moderate drinking, HED, and HID on co-use days.

Methods: Young adults who reported simultaneous alcohol and cannabis use (N = 318) from three US universities completed five repeated daily surveys for 54 days reporting number of drinks and number of cannabis uses (i.e., frequency/events), forms, and quantity.

Results: Participants reported increased likelihood of HID and HED versus moderate drinking on co-use versus alcohol-only days. HID versus HED did not differ between co-use and alcohol-only days. On co-use days, heavier drinking was more likely on days with more cannabis use. The form of cannabis used and the use of multiple versus single forms of cannabis were not associated with drinking level. Yet, HID was more likely than HED and moderate drinking when more grams of flower were used and HID was also more likely than moderate drinking when more hits of concentrates were used.

Conclusions: Findings show heavy drinking (HID, HED) was more likely than moderate drinking on co-use days and cannabis use characteristics may influence drinking levels. Co-use interventions may benefit from a focus on heavy drinking and cannabis use characteristics.