One may not eat or drink after eating the Afikomen, there is a requirement that the taste of the Afikomen matzo last in our mouths (see Shulchan Aruch 481:1)
Q. May one gargle with mouthwash before he goes to sleep [or for those who permit brushing teeth on Shabbos and Yom Tov - as do many S'fardim - may they brush after the Seder before going to sleep ?
Q. May one take Tums after the completion of the seder if he is experiencing Heart burn triggered by all the wine and matzos consumed by the seder ?
Rosh Hayeshiva: The answer revolves around understanding why one may not eat or drink after the Afikomen. After fully understanding why we don't eat or drink we could clarify what may or may not be put into our mouths after the seder.
The Rambam (Chometz Umatzo chap. 8:9) gives the reason for the Halacha of not eating or drinking after the Afikomon in order that the taste of the matzo (commemorating the Korban Pesach) will stay in our mouth.
One can ask, when we say that we want the taste of the afikomen to stay in the mouth, does "Taste In The Mouth" mean solely the mouth —or does it mean the throat and stomach as well ?
If indeed the issue is not merely the mouth but the "entire digestive system" maybe just rinsing the mouth or brushing the teeth would not fall under this prohibition. For only eating and drinking washes the throat and stomach. But on the other hand taking a flavored Tum would indeed be a problem for it would definitely remove the taste of the matzo from both the mouth, as well as the throat and stomach.
According to the Ramban (Milchamos -Pessachim 26:2) the reason for the prohibition of eating after Afikomen is so that the Afikoman is eaten on a full stomach to an extent that one doesn't have room to eat anything else after. (If the Chazal would permit eating after the Afikomen one may possibly not fully fill himself, giving himself room to snack later) According to Ramban's reason both brushing/gargling and taking Tums should not be an issue.
According to Bal Hamaoar (ibid) we are concerned that eating will continue and the Hallel will be pushed off — and even possibly skipped all together —especially in the manner the Hallel was recited in the olden times (where the assembled "would leave the table" and ascend to the roof tops of their homes and say the Hallel). According to this reason as well, both gargling and or brushing before going to bed or taking Tums after the seder has ended would not be of issue.
According to Rabainu Yonah the reason for the prohibition is to eliminate the possibility that one will continue to indulge with wine [and food] causing him to fall asleep and then not continue retelling the story of Yetzias Mitzraim through out the continuation of the entire night. According to this explanation too there would be no reason to forbid gargling / brushing nor the taking of Tums.
There is also a prohibition of drinking after the Afikomen which is possibly separate from the prohibition of eating after the Afikomen (See Mishna Berura 481:1 as it is explained there that possibly the reason of not drinking after the Afikomen is not the same as eating after the Afikomen, and can be an independent prohobition)
The prohibition of drinking after the Afikomen and its reason is relevant to our discussion. Some say that the prohibition of not drinking after eating the Afikomen refers strictly to wine for we are afraid of intoxication and then the person will not continue with the Hallel and the Seder. According to this reason the problem would be strictly with wine or other intoxicating beverages. Mouthwash, Tums or brushing teeth are of no concern.
One may ask, if we are concerned that the taste of the Afikomen remain and therefore we forbid eating, why then is drinking all beverages besides wine not a problem? The answer we must say is that chewing and consuming food evicts taste more then drink. If so we still may not gargle for mouth wash is strong and is designed to evict taste.
Yet others say that the problem with drinking after the Afikomen is that it looks like one is "Adding to the 4 cups" of wine. According to these opinions only drinks (Chamar Medina) would be a problem. (As it looks like one is starting a new Seudah. See Mishna Berurah 481:1 as any Chamber Medina is technically acceptable to make Kiddush)
Yet according to many the reason for not drinking after the Afikomen is indeed to preserve the taste of the matzo. Are we to include gargling and brushing?
Let us add to the conversation that the poskim, including the Mishna Berura (481:1) permit drinking water or tea — probably this includes coffee to, though the M.B doesn't mention it. [some even permit tea and coffee with sugar - see Chazon Ovadya (Pesach vol.2 pg 112) though in the Mishna Berura "The Velt learns" that he only permitted tea (and coffee) without sugar. This is difficult to accept for he clearly permits drinking "Apple Trenk [cider or compote Be'er Heitiv (481:1)]" which is juice and definitely has sugar]
Yet possibly "Mouth Wash" maybe worse then tea and coffee, for it is usually made from strong antibacterial ingredients such as the case with Listerene. [Notice though that the Mishna Berura (ibid) permitted drinking "Ingber Water" which seems to be ginger. As it is know that ginger is quite strong and potent. [I believe that it was also used years ago as a mouth wash and for gargling agent] It is hard to see the difference between ginger water and mouth wash.
We may add to the conversation that the Be'er Heitiv (481:1) permits smoking after the Afikomen [its possible though he is not referring to smoking for he uses the language "Shtiyas" drinking the Tobacco "Tabac"] There is no doubt that smoking will erase the taste of matzo from the mouth [please note we are not condoning smoking on Yom Tov in todays times—nor for that matter at any other day]
Also important to add, that it is the opinion of the Chida (in Chaim L'rosh) that the prohibition of eating and drinking after the Afikomen maybe only until Chatzos —midnight. After that all eating and drinking becomes permitted.
There may be another issue with taking Tums after the seder for it is prohibited to take medicine on Yom Tov. Chazal forbade the consumption of medicine on Shabbos. On Yom Tov though, there is a Machlokes Haposkim, Shemiras Shabbos Kehilhasa (33:25) says its similar to Shabbos and is Assur. Rav Elyashiv (Nishmas Avrohom Vol.1 691) says there is room to be lenient. Hacham Ovadya Yosef (Chazon Ovadya - Yom Tov 7) concludes that it is permitted. Though Tums is merely a safe over the counter remedy and consumed frequently as "calcium supplement", may nevertheless still be regarded as a medicine and may have the issue taking medicine on Yom Tov. (According to the Shmiras Shabbos Kehilhasa)
Yet maybe one who has heart burn can be included in the category of "Choleh Shein Bo Sakana" that the person is considered a "Nofel Lmishkav". Many people don't tolerate severe heart burn well, it makes them completely sick.
[It is important to note that the second day "Yom Tov Shaini D'Golius" is more lenient in regards to the taking of Medicines (See Mishna Berurah 494:5-7]
In Conclusion: One may gargle / brush before he goes to bed. Taking Tums should be avoided unless the heart burn is severe. Yet if need be, preferably it should be taken after Chatzos - midnight. If possible non flavored antacids are preferable over flavored ones.