Giving Halachic rulings under the influence of alcohol or other substance:
Every person must beware not to give a Halachic ruling when under the influence of wine or other alcoholic beverages [or even non-alcoholic beverage which has slightly blurred his mind]. [The same applies for one who is in a state of distress, that he should not give a Halachic ruling at this time. It is a Midas Chassidus for every Rav to abstain from ruling on a day that he was angered, or arrived home from travel, even though this is permitted from the letter of the law.]
Level of intoxication: It is forbidden to give a Halachic ruling as soon as one feels even slightly intoxicated, or one drank a Revius of wine or another alcoholic beverage. [Practically, however, by today’s wines, if one does not feel intoxicated at all he may continue giving rulings even if he drank a Revius or more.]
The length of time one must wait prior to ruling: The prohibition against ruling while under the influence extends until one feels that the intoxication has subsided. [Some Poskim rule that by large feasts, such as on Yom Tov, Purim, Millah, wedding, it is forbidden to rule until the next morning. Practically, however, he may rule as soon as he feels for certain that the intoxication has subsided.]
What complexity of rulings does the prohibition include? This prohibition applies even against giving a ruling regarding a simple matter, unless the matter is explicit in Poskim and is something that even children know. This implies that so long as something is explicit in Poskim, even if it is not explicit in the Chumash, it is permitted to rule on it while drunk. However, practically, it is forbidden to rule on any matter that is not explicitly written in the Torah, even if it is explicitly written in Poskim. The measurement is as follows: If the matter is so explicit in the Torah that even the Tzedokim agree to it, then one may rule on it even when drunk, and if not then it is forbidden. Thus, for example, he may say that a Sheretz and frog are pure and blood is forbidden, but may not say the rule of Nosein Taam Lepegam, and Batul Beshishim.
What topics of ruling does the prohibition include: Some Poskim rule that the entire prohibition is only limited to Issur Viheter, while by other subjects, such as Choshen Mishpat, it is permitted to rule. Other Poskim however rule that it applies to all subjects.
Learning Torah: It is permitted for one who is drunk to learn Torah, even Halachos and Midrashos, so long as he does not give any rulings. If, however he is a Posek, then he is not to teach others, as his learning is a ruling in it of itself.
 Rama Y.D. 242/13; Eiruvin 64; Kerisus 13b; Kesubos 10b; Nazir 38a; Rambam Bias Hamikdash 1; Rashi Shemini; Terumos Hadeshen 42; Maharik Shoresh 170;
 Shach 242/20; Rambam ibid “Such as dates or milk”
 Shach 242/21; Bach 242
 Shach ibid; Tosafus in name of Yerushalmi
 See Shach 242/19 based on Rambam and Gemara ibid that even a mere Revius invalidates him from ruling. See also Admur O.C. 99/1-4; Practically, however based on the ruling of Poskim in next footnote that today’s wines are no longer similar to the wines of back then, a Revius no longer automatically invalidates oneself, and it is all dependent on if one feels intoxicated or not.
 Smeh C.M. 7/11; Terumos Hadeshen 42; Admur 99/4 regarding Davening; See previous footnote
 Admur O.C. 99/4 regarding Davening “It suffices to wait until he no longer feels intoxicated”; Beis Yosef C.M. 7 rules “As soon as he fells the intoxication has subsided he may rule”; See Shach 242/19 based on Rambam and Gemara ibid that if he drank a mere Revius he cannot rule until he sleeps, walks a Mil [if he drank exactly a Revius]. If he drank more than a Revius then he must wait until the intoxication completely subsides. See also Admur 99/1-4 for the differentiation between one who drank a Revius and one who drank more than a Revius.
 Rashba 247
 Shach 242/19; Beis Yosef ibid
 Rama Y.D. 242/13; Terumos Hadeshen 42; Maharik Shoresh 170
 Rama ibid; Literally “Zil Kari Beir Rav” which means that one can go to Cheder and learn it. [See Horiyos 4a]
 Shach 242/21 that so is implied from Rama ibid and Mahrik ibid, however see conclusion of Shach ibid that perhaps there is a misprint in Rama and instead of Poskim it should say Pesukim; See also Taz ibid who seems to imply that even the Rama holds like the ruling brought next.
 Shach 242/21; Taz 242/3; Terumos Hadeshen ibid; Rambam ibid
 Taz ibid; Terumos Hadeshen ibid
 Michaber C.M. 7/5 “Some opinions rule a drunk person may arbitrate monetary cases”; Smeh 7/11; Tosafus Sanhedrin 42a; Shvus Yaakov 1/140, brought in Pischeiy Teshuvah 242/6
 Teshuvos Habach 41, brought in Pischeiy Teshuvah ibid
 Shach 242/19; Bach ibid; Rambam ibid; Kerisus ibid