Twenty-first birthdays are associated with heavier drinking and more negative consequences than any other high-risk drinking event. Friends are the strongest social influence on young adult drinking; however, previous research on college students’ drinking has often only examined individuals’ perceptions of “friends” generally. Unfortunately, this may obscure the positive influence of some friends and the negative influence of others. Using data drawn from a larger intervention study aimed at reducing 21st birthday drinking, this research examined how specific friends (N = 166) who were present at 21st birthday celebrations may have exacerbated or mitigated celebrants’ (N = 166) experience of alcohol-related consequences, as well as how characteristics of that friendship moderate these effects. Controlling for sex, alcohol consumption, and friend prointoxication intentions for the celebrants’ 21st birthday drinking, higher friend prosafety/support intentions predicted the celebrants experiencing fewer alcohol-related consequences. Higher prosafety/support intentions also buffered participants from the negative influence of friend prointoxication intentions. Furthermore, the closeness of the friendship moderated this effect. At high levels of closeness, having a friend with lower prosafety/support intentions was associated with more alcohol-related consequences for the celebrant. Post hoc analyses revealed that this effect may have been driven by discrepancies between celebrants’ and friends’ reports of friendship closeness; celebrants’ perception of closeness that was higher than the friends’ perception was associated with the celebrant experiencing more alcohol-related consequences. Results demonstrate the ways that specific friends can both mitigate and exacerbate 21st birthday alcohol-related consequences. The implications of the present findings for incorporating specific friends into drinking-related interventions are discussed.