(This should not be relied upon for practical halacha. When a question arises a Rabbi should be consulted.)
Fish and Dairy-
1. Harav Yosef Karo zt”l in the Beis Yosef (Yoreh Deah 87), the Shulchan Aruch’s commentary on the Tur, writes that “one should not eat fish and milk together because of the danger involved, as it is explained in O.C. 173.”
2. However, many authorities, including the Rama, Prisha, Taz, Shach, Magen Avraham and Aruch Hashulchan point out that the location the Beis Yosef referenced for his halachic decision to be machmir is referring to eating fish with meat, not milk. They therefore maintain that this issue is a case of mistaken identity (misprint) and that eating fish with milk is 100% permissible. The Chida (Machzik Bracha 87:4) also feels that it is permissible to eat fish and dairy together and that there is a misprint in the Beis Yosef. He adds that if it is indeed true that fish and dairy is dangerous, Harav Yosef Karo should have written so in his Shulchan Aruch.
3. It is worth noting that although many assume that there was a misprint in the Beis Yosef, the notion of avoiding fish and dairy due to danger was already advanced hundreds of years earlier by Rabbeinu Bachya (Shemos 23:19). Rabbeinu Bachya writes, “The doctors feel that fish and cheese that are cooked together can cause bad health and tza’raas.”
4. Indeed, there were authorities who were stringent and did not allow for fish to be eaten with dairy. They explain that they also heard from doctors that fish and dairy can be harmful to one’s health (see Darkei Teshuva 87:43 and Yechava Daas 6:48).
5. Other authorities, including the Chinuch Beis Yehuda, Pri Megadim (see Pischei Teshuva Y.D. 87) and Kaf Hachaim (87:24), differentiate between fish and milk (or cheese) which they believe to be harmful and fish and butter (or cream) which is not damaging to one’s health. The Ben Ish Chai (Year 2 Bahaloscha 15) cites these authorities and writes that one should be stringent with all forms of dairy and that in Baghdad the common custom is to refrain from even eating fish and butter.
6. Harav Ovadia Yosef zt”l discusses this issue at length and concludes that, “One should refrain from eating fish and milk or cheese due to the potential danger involved. However, those that are lenient to eat fish and butter together are permitted to continue in their approach. Ashkenazi Jews dismiss this issue entirely and eat fish with all forms of dairy and they have what to rely on.” It is based upon the ruling of Harav Ovadia Yosef zt”l and the Ben Ish Chai that many Sefardim avoid eating fish and dairy together (some will eat fish and butter, as cited above). However, as noted most Ashkenazic communities dismiss this issue entirely.
Drinking Water After Fish-
1. The Rama (Y.D. 115:3) writes that in between eating fish and meat one should eat and drink something.
2. Tosafos (Moed Katan 11a) writes that in his day it was considered dangerous (to one’s heart) to drink water immediately after eating fish. This teaching is cited by many authorities, including Rav Akiva Eiger zt”l (Y.D. 116:5) and the Aruch Hashulchan.(116:10)
3. According to Harav Nissam Karelitz shlit”a (Chut Shani Shabbos vol. 4 page 399) tea, coffee, soda and cola have the same status as water.
4. Harav Nissam Karelitz shlit”a writes that today one may rule leniently. He explains that Tosafos never meant to imply that this is a rabbinic prohibition, rather the doctors of his time felt that it was harmful to drink water after eating fish. Therefore, today where the doctors feel that it is no longer harmful one may rule leniently. Harav Karelitz shlit”a continues that some are stringent however, and avoid drinking water after eating fish.
5. On Shabbos, following the fish course, those who are stringent face an interesting dilemma. As noted above (halacha 1) in between fish and meat one should drink something. However, according to Tosafos one should not drink water (or soda etc.) after fish. Therefore, writes Harav Karelitz shlit”a many have the custom to drink wine or whiskey in between the fish and chicken soup courses.
6. Harav Chaim Elazar Shapiro zt”l of Munkatsh (the author of the Minchas Elazar) added another reason why many drink whiskey immediately following the fish course on Friday night. The word whiskey in Hebrew is “yayin saraf.” The first two letters being “yud” and “shin.” The first letter of the hebrew word for fish is “daled” (“dagim”). All three letters together spell out the name of Hashem “Shakay.” In order not to separate the name of Hashem one should drink the whiskey immediately after the fish (Darkei Chayim V’Shalom 396).
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