We have learned that the Sages ordained the afikoman at the end of the Seder to commemorate the korban Pesaḥ, which was eaten “while satisfied.” Just as it was forbidden to eat any other food after the Paschal sacrifice, so that its taste lingered, so too the Sages forbade eating after the afikoman.
It is also forbidden to drink wine after the afikoman, for several reasons. If one has not yet finished reciting the Hagada, he might become intoxicated and be unable to finish reciting Hallel properly; furthermore, by drinking an additional cup of wine, one will appear to be adding to the number of cups instituted by the Sages.
It is even forbidden to drink wine after the Seder, because there is a mitzva to delve into the laws of Pesaḥ and the Exodus story until one is overcome with sleep, and if one drinks wine or some other intoxicating beverage, he will not be able to do so (Rabbeinu Yona, Rosh). Furthermore, even though the Seder is over, if one drinks additional cups of wine he may still appear to be adding to the cups instituted by the Sages, or starting a new series of cups (Ramban, Ran).
Some poskim maintain that it is even forbidden to drink coffee or juice after the afikoman until one goes to sleep, because any flavored food or drink weakens the taste of matza in one’s mouth, so just as it is forbidden to eat after the afikoman, so is it forbidden to drink anything flavored. To be sure, we drink two more cups of wine after the afikoman, but since these are part of the mitzva, they are not deemed to weaken the taste of the mitzva. According to this view, only water may be drunk after the Seder.
In contrast, many poskim permit drinking coffee or juice, for only food is deemed to weaken the taste of the afikomen; beverages are not included in the prohibition.
In practice, if one wishes to drink coffee or juice after the Seder, he may, as this is the view of most poskim. Le-khatḥila, though, one should be stringent and avoid drinking anything except water. If one wishes to drink coffee so that he will be able to continue delving into the laws of Pesaḥ and the Exodus story, he may do so even le-khatḥila.
. According to Ha-ma’or and several other Rishonim, once the Seder has ended, one may drink more wine, even if it will cause him to become intoxicated. Conversely, Mordechai and Hagahot Maimoniyot record a view that even drinking water is forbidden. Most Rishonim maintain that the prohibition is to drink wine or ḥamar medina, lest he become intoxicated and be unable to continue delving into Pesaḥ topics (Rosh), or lest he appear to be adding on to the four cups (Ramban). Some poskim follow Rif and Mahari Weil, who maintain that one is only permitted to drink water, since water has no taste and will not ruin the aftertaste of the afikoman. Accordingly, Knesset Ha-gedola and Ma’amar Mordechai prohibit drinking coffee or other flavored drinks after the Seder. In contrast, some Aḥaronim maintain that that even according to Mahari Weil one may drink a beverage with a weak taste, and the prohibition is only on drinking beverages that have a strong flavor. So states MA 481:1. According to this, the status of coffee requires clarification. Regardless, according to most Rishonim, one may drink coffee or juice, since these drinks do not intoxicate and drinking them does not look like adding to the four cups. It is attested that Ḥatam Sofer would drink coffee after the Seder every year. According to SAH (481:1), MB (481:1), and Ben Ish Ḥai (Tzav 35), one should preferably be stringent and refrain from drinking coffee and flavored drinks, but they permit it when there is a great need. As I have written, if by drinking coffee one will be able to study Torah, he should certainly drink coffee, even le-khatḥila.
According to Ḥok Yaakov 481:1, since the main reason not to drink strong beverages is to avoid drowsiness, once one is drowsy he may even drink alcoholic beverages. Many Aḥaronim cite this view. SAH (481:1) also quotes this opinion, and adds that according to those who prohibit drinking because the beverages ruin the taste of the afikoman, the prohibition applies all night. Accordingly, it is clear that one may not eat throughout the night, since that would certainly ruin the aftertaste of the afikoman. In an extreme situation, one may be lenient based on the opinion of Avnei Nezer (OḤ 381), which maintains that according to R. Elazar b. Azarya, since the time for eating the korban Pesaḥ (and the afikoman) ends at midnight, the prohibition against eating afterward ends at midnight as well. Therefore, in extenuating circumstances, since this is only a rabbinic injunction, one may rely on Avnei Nezer’s explanation of R. Elazar b. Azarya’s view.
According to the custom of Sephardim, who do not recite a berakha on the fourth cup of wine, one who wishes to drink water between the third and fourth cups does not recite a berakha on it (provided that he had in mind to drink water or the water was in front of him), since the berakha on the third cup covers the fourth cup and any other beverage. According to Ashkenazic custom, the berakha on the third cup does not cover the fourth cup; therefore, if he already poured the fourth cup, even if the water is already in front of him, he must recite a berakha (“she-hakol”) before drinking it, as he has already been distracted from the “ha-gefen” that he made on the third cup.