Halacha forbids Kohanim from reciting Birkat Kohanim after drinking wine. If a Kohen did drink wine and it then came time for Birkat Kohanim, he should leave the sanctuary before Birkat Kohanim.
The Shulhan Aruch (128:38) rules that a Kohen becomes disqualified from reciting Birkat Kohanim if he drinks a Rebi’it (approx. 3.1 ounces) of undiluted wine all at once, or more than a Rebi’it of either diluted or undiluted wine even in several sips. If a Kohen drinks more than a Rebi’it of wine, he may not recite Birkat Kohanim until the wine is out of his system, even if it was diluted and even if he drank in several sips.
Hacham David Yosef, in his Halacha Berura (p. 473), cites a debate among the Halachic authorities regarding the status of other alcoholic beverages in this regard. According to some authorities, a Kohen may recite Birkat Kohanim after drinking other alcoholic beverages – such as scotch and arak – as long as he has not reached the point of complete inebriation ("Shichruto Shel Lot") such that he cannot even recognize his family members. Even if he is clearly intoxicated and cannot pronounce his words properly, he may nevertheless recite Birkat Kohanim. Others, however, disagree, and maintain that a Kohen may recite Birkat Kohanim only if he is able to present himself in a befitting manner when approaching a king. And thus if he slurs his words or cannot stand up straight, he may not recite Birkat Kohanim.
Hacham David rules that one should follow the stringent view, and thus even if a Kohen drank a beverage other than wine, he should not recite Birkat Kohanim unless he is able to speak properly and present himself in a dignified manner.
Hacham David further notes that wine which is Mebushal (had been boiled) has the status of other alcoholic beverages in this regard. Therefore, even if a Kohen drank more than a Rebi’it of Mebushal wine, he may still recite Birkat Kohanim if he is able to present himself properly.
This question commonly arises on Shabbat and holidays when a Kiddush is held in between Shaharit and Musaf. Kohanim must ensure not to drink a Rebi’it of non-Mebushal wine before Musaf, for if they do, they are disqualified from reciting Birkat Kohanim. If a Kohen wants to drink a Rebi’it, he should either drink grape juice or use Mebushal wine and ensure not to become inebriated.
Summary: A Kohen who drinks a Rebi’it (3.1 oz.) of wine all at once, or more than a Rebi’it of wine in several sips, may not recite Birkat Kohanim until the wine is out of his system. Therefore, if a Kohen is making Kiddush before Musaf, he should either use grape juice, or use Mebushal wine, in which case he may recite Birkat Kohanim even if he drinks a Rebi’it, as long as he is able to speak and present himself in a proper, dignified manner. A Kohen who drinks alcoholic beverages other than wine may recite Birkat Kohanim if he is able to speak and conduct himself in a proper, dignified manner.
