May one eat, drink, and say Lechaim, while wearing Tefillin?
A meal: One who enters [while wearing Tefillin] to eat a set meal [i.e. bread] is required to remove his Tefillin. He is to leave them on the table so they are available to him after the meal is complete, in order so he return and wear them prior to Birchas Hamazon.
A snack: One may eat a temporary meal [i.e. snack] while wearing his Tefillin, and there is no need to remove them beforehand. [Certainly, one may drink tea or coffee while wearing Tefillin. Some Poskim, however, question that perhaps this only applied during the times that one would wear the Tefillin throughout the entire day. However, in today’s times that the Tefillin is only worn during the prayers, it would be forbidden to eat even a snack while wearing Tefillin. Practically, however, the implication of the Poskim is that it is permitted even today and so is the final ruling. Nonetheless, if one anyways plans to already remove the Tefillin, then it is proper for it to be remove before the eating.]
It is forbidden to eat a meal with one’s Tefillin, although a mere snack may be eaten.
What is defined as a snack and what is defined as a meal?
The definition of a snack and a meal are the same as defined regarding the obligation to eat in a Sukkah. Thus, a meal is defined as more than a Kebeitza [55cc or 55 grams] of Mezonos or Hamotzi, and hence this amount may not be eaten while wearing one’s Tefillin. [However, less than 55 grams of Mezonos or Hamotzi is defined as a snack and may hence be eaten while wearing Tefillin. Likewise, all Borei Nefashos foods and drinks are defined as a snack and hence one may eat any amount of them, and drink any amount of non-alcoholic drinks while wearing Tefillin.]
May one drink Lechaim [alcohol] while wearing Tefillin?
It is permitted to drink a small amount of Lechaim that will not get one tipsy, while wearing Tefillin, although only up until a Kebeitza [55 cc]. Some Poskim, however negate the drinking of any alcohol while wearing Tefillin.
May one smoke cigarettes while wearing Tefillin?
One is to avoid smoking cigarettes while wearing Tefillin in public due to it being belittling to the Tefillin. In private, however, one may be lenient. As well if one feels he needs to smoke in order to clear his mind for Torah learning with his Tefillin, then it is allowed to smoke even in public.
Must the blessing be repeated if one ate a set meal after removing the Tefillin, and then re-wore them?
Some Poskim question whether the blessing must be repeated in such a case. Practically, one is to use the bathroom in between and then re-wear it with a blessing. [If, however, one removed the Tefillin for the sake of eating a mere snack, then the blessing is not to be repeated.]
 Admur 40:11; Michaber 40:8; Brachos 23b; See Piskeiy Teshuvos 40:8
 The reason: It is forbidden to eat a meal with one’s Tefillin due to worry that one may come to get drunk and belittle [himself] while wearing his Tefillin. [Admur ibid]
 Admur ibid; Michaber ibid; Yerushalmi Brachos 1:3
Other opinions: The Mishneh Berurah [140:18] brings opinions which question whether this allowance to eat a snack applies even today when we no longer wear our Tefillin throughout the day. In Piskeiy Teshuvas [40:8] he rules [based on the M”B] that if one plans to remove his Tefillin already, then he is not to eat or drink at all while wearing them. If however he plans to wear them and thus desires to eat and drink now in order to have strength to learn or pray, then he may do so, and so was the custom of Gedolei Yisrael [Chasam Sofer; Mahrahm Shick].
 The reason: It As by a mere snack there is no reason to suspect that he may come to get drunk and belittle [himself] while wearing his Tefillin. [Admur ibid]
 Kaf Hachaim 40:35
 Shulchan Shlomo, brought in M”B 40:19
 Over Oreiach in name of Adart; Dinim Vehanhagos 3:15 in name of Chazon Ish; Teshuras Shaiy 1:215; See Piskeiy Teshuvos 40:8 that so was the custom of Gedolei Yisrael, including the Chasam Sofer and Maharam Shick
 Piskeiy Teshuvos ibid
 Mateh Yehuda 40:9; Erech Hashulchan 40:5; Pischei Olam 401:5; M”B 40:20; Kaf Hachaim 40:34; Ketzos nHashulchan 8 footnote 54; Implication of opinion of Admur who uses the terms of Keva and Aray which are also used by Sukkah.
Other opinions: Some Poskim rule that one may eat up until Kevius Seudah of Mezonos, which is up to 4 Kebeiytzim. [Afikei Maginim Biurim 18; Keser Rosh 16; Piskeiy Teshuvos 40:8]
 In this matter, one is to be stringent to measure in volume and not weight, if the food item is a lesser weight than water and hence floats on it [i.e. crackers/Matzah]. However, by regular bread, one can measure in weight [i.e. 55 grams], as it is similar to its volume. [See Shiureiy Torah 3:12-13 and p. 184]
 Piskeiy Teshuvos 40:8
 Implication of Admur ibid who rules to wear the Tefillin prior to Birchas Hamazon, even though one recites a blessing on the Kos Shel Bracha afterwards; M”A 232:17 regarding prior to Mincha that one may drink up until a Kibeitza and so rules Kaf Hachaim 40:35; Piskeiy Teshuvos 40:8; Daas Noteh 832 of Rav Chaim Kanievsky
 Sifsei Nevonim 31; Rav Yaakov Yosef; See Ohel Yaakov p. 186
 The reason: As this is the entire reason for why eating a meal is prohibited with Tefillin.
 Rivivos Efraim 1:30 in name of Peir Yaakov 4; Piskeiy Teshuvos 40:8
 Biur Halacha 40:8 based on Yerushalmi