Several authorities, including the Rif and Tur,[5] when codifying this Mitzvah, do indeed use the basic understanding of the Gemara’s ruling, that one is required to get so drunk on Purim that he cannot tell the difference between ‘Cursed is Haman’ and ‘Blessed is Mordechai’, implying quite drunk.

Yet, Rabbeinu Efraim, cited l’halacha by the Ran and Baal HaMa’or,[6] rules the exact opposite! He maintains that since the Gemara tells the story of Rabba and Rabbi Zeira after the ruling of getting drunk, it is not meant exclusively as a cautionary tale detailing the evils of excessive alcohol imbibement; rather, it is coming to negate the ruling! According to this understanding, it is forbidden to get drunk on Purim!
V’lo Ad B’Chlal!
A different explanation of the Gemara is that drinking “ad d’lo yada bein Arur Haman L’Boruch Mordechai” does not actually mean getting stone-cold drunk. In fact, most commentaries offer many different rationales as to the Gemara’s intent with this phrase.

Some say it means drinking until one can no longer perform the mental acrobatics necessary to be able to add up the Gematria of Arur Haman andBaruch Mordechai [7] (Hint: they both equal 502!) (R. Abraham Abele ben Hayyim haLevi Gombiner.). Accordingly, this is a much lesser degree of drunkenness. Others explain it means drinking until one can no longer decide which was a greater miracle: the downfall of Haman or Mordechai’s meteoric rise in prominence.[8] Another interpretation is to drink enough to no longer be able to recite a lengthy Purim-themed Alef-Bais acrostic poem in the proper order.[9] An additional understanding is that one must get inebriated just enough to no longer be able to properly thank Hashem for the many miracles of our salvation Purim time.[10]

The opposite approach is attributed to the late 11th-century North African halakhist Rabbenu Ephraim ibn Avi Alragan, “Based on the story of Rabbah getting up and slaughtering R. Zeira, we reject the statement of Rava, and it is wrong to act in such a way” (quoted by the Ra”n, ad. loc.).
Maimonides, however, replaces the “can’t tell the difference between Haman and Mordecai” standard with one that is more easily appraised:
How does one fulfill the obligation of the Purim Seudah? One should eat meat and prepare as nice a meal as one can afford and drink wine until one becomes drunk and falls asleep from drunkenness. (Laws of Megillah 2:15)

Maimonides’ reading finds substantial support in the comments of the 16th-century Talmud commentator R. Samuel Eliezer ben Judah haLevi Edels, better known as the Maharsha. The Maharsha understands the story of Rabbah and R. Zeira as an example of figurative language:
It is surprising to explain this as if Rabbah actually committed this crime, but rather, it is as if Rabbah butchered him. That is to say, Rabbah urged R. Zeira to drink too much until he got sick and almost died… The words “brought him back to life” in both the Bible and the Talmud can simply mean healing. If the Maharsha’s reading underlies Maimonides’ ruling that one should drink until one passes out, then perhaps, against the opinion of Rabbenu Ephraim ibn Avi Alragan, the story of Rabbah and R. Zeira is meant to be indicative of normative practice (the drinking, not the swordplay).