Fact: There is a solid halachic basis for using kosher soap exclusively.
Background: People often fail to consider what a marvelous substance soap is—when used with water, it loosens unwanted particles, emulsifies grease and absorbs dirt into foam. Its efficacy is usually due to a chemical reaction between an acid and a base. Typically, the acid derives from either vegetable or animal fat. Historically, most soaps were made from animal fats. Even today, excluding “vegetarian” soap, most soaps are made from animal fat. For example, Ivory soap (advertised as “99 44/100% pure”) is, according to its web site, “made of both vegetable oils and animal fats.” In response to my telephone inquiry, the Procter & Gamble spokesman stated that the animal fats in Ivory are the usually discarded tallow of beef and pork. In addition, animal fats are the main ingredients in Ivory, making up over fifty percent of the product.
All fats from non-kosher animals, and certain fats known as chelev from kosher animals, are prohibited for consumption. The Talmud (Pesachim 21-23) and legal codes (Rambam, Ma’achalot Asurot 8:15) explain that while it is forbidden to eat these fats, it is permissible to derive other benefits from them.
As an aside, there are instances when it is forbidden to derive benefit from non-kosher items. Thus, for example, the Gemara (Shabbat 28b) states that tefillin straps should only be made from kosher animals. This principle, called mutar beficha, is codified in the Shulchan Aruch (OC 32:12). The Magen Avraham (586:3) postulates that in regard to this principle, all mitzvot should be patterned after tefillin, and thus, non-kosher items should not be used to fulfill any mitzvah. Most others, however, contend that the prohibition applies only to tefillin straps. Thus, for example, the Shu”t Beit Shlomo (OC 108-109) permits using candles made from non-kosher fat for Chanukah and Shabbat. It seems that the prevailing custom is not to follow the stringency of the Magen Avraham.
The Mishnah (Yoma 73b) presents a potential problem regarding the use of non-kosher soap. Among the pleasurable acts that the Mishnah lists as being prohibited on Yom Kippur is anointing oneself. This is based on a verse (Psalms 109:18) from which the rabbis determined that “sichah keshtiyah, anointing is tantamount to drinking” (Mishnah, Shabbat 86a; Yoma 76b). Because applying soap is considered “anointing,” some authorities compare using soap made from non-kosher fats to drinking a nonkosher beverage (see middle of Taz, YD 117:4). They therefore prohibited using such soap. Of course the comparison between anointing and drinking is not absolute. It would be ludicrous to assert that after making Kiddush on Friday night, in lieu of drinking the wine, one need only anoint oneself with it. Yet the comparison may be strong enough to warrant the prohibition of anointing oneself with non-kosher fats.
Those who permit soaps made of non-kosher animal byproducts base their ruling on the position of Tosafot (Niddah 32a, s.v. ukeshemen) who assert that sichah keshtiyah only applies to oil but not to fat or to soaps made from fat. Furthermore, they (Yoma 77a, s.v. minayin) assert that sichah keshtiyah is rabbinic in nature and that the rabbis only applied it to specific cases such as Yom Kippur. According to Tosafot, one is not permitted to anoint oneself on Yom Kippur, but one may anoint oneself with any kind of soap or fat during the rest of the year.
The position of the Beit Yosef is less clear. While he cites both the Orchot Chaim who maintains that one may not anoint oneself with non-kosher fats (YD 117) and the Rashba who has a more lenient view (YD 123), the Beit Yosef does not comment on either opinion. The Shach (Nekudat Hakesef, YD 117) cites many authorities who permit using non-kosher soap. Still there are others who do not permit it. The Kaf Hachaim (YD 117:15) presents both sides in a lengthy discussion but concludes by taking a stringent position. The Taz (middle of YD 117:4), citing the Issur veHetter, permits the use of soaps made of nonkosher animal byproducts only to relieve discomfort. He stresses that a healthy individual should not use such soaps for pleasure. The Shulchan Aruch, in the section on the laws of Shabbat (OC 326:8), rules that one may not use soap on Shabbat because of the prohibition against “smoothing.” Because the Shulchan Aruch does not distinguish between kosher and non-kosher soap, the implication is that using any soap is acceptable during the rest of the week. The Vilna Gaon rejects this lenient view and cites many early authorities who disagree with the reasoning of the Tosafot. The Vilna Gaon therefore considers using non-kosher soap as tantamount to drinking a non-kosher beverage. The Aruch Hashulchan (YD 117:29) notes that it seems that “everyone uses non-kosher soap without rabbinic objection.” He suggests that even the Vilna Gaon would agree that today it is permissible to use non-kosher soap, since nowadays soaps taste awful and are not even raui l’achilat kelev, fit for animal consumption. Because it is inedible, there would be no prohibition against drinking it, and thus sichah keshtiyah would not apply.
The Chofetz Chaim (Biur Halachah, end of OC 326), after citing both the Vilna Gaon’s stringent position and Tosafot’s more lenient one, observes that the custom is to be lenient. However, he continues, if kosher soap is available one should try to be stringent and use it exclusively. In a similar vein, the Darchei Teshuvah (YD 117:33) notes that since quality kosher soap is available at a reasonable price, it is preferable for one to use it. Two contemporary authorities, Rav Moshe Stern (Be’er Moshe, 3:87) and Rav Ovadiah Yosef (Yechave Da’at, 4:43) agree that because today’s soaps made from prohibited fats have such unpleasant tastes and are inedible, they may be used.
There are many soaps today in which tallow is only a minority ingredient and still others in which the presence of tallow is difficult to determine. According to some authorities, there is a basis for leniency in both of these cases.,  It is also important to note that many liquid soaps are actually detergent, not soap, and therefore they may not contain any animal fat at all.
1. Soap may have already been used in the Biblical period. The word “borit” in Jeremiah 2:22 and Malachi 3:2 is understood by many to refer to a kind of soap. It is possible that in the Biblical and Talmudic periods, soap was derived almost exclusively from plants. See Rashi to Malachi 3:2 and Encyclopaedia Judaica 15:18-19.
2. A registered name. Interestingly, the soap’s name is based on Psalms 45:8: “All thy garments smell of myrrh and aloes and cassia, out of the ivory palaces whereby they have made thee glad.”
3. Based on Exodus 13:9.
4. The boxes, hairs and sinews of tefillin also must only be made from a kosher animal, but this is a halachah leMoshe miSinai and not derived from the verse in Exodus. See also Shabbat 108a and Sofrim 1:1.
5. See Noda B’Yehuda Tinyana (chelek 2, siman 3) who has a long discussion of this principle. He discusses issues such as polishing tefillin straps with non-kosher oil. He also questions how the Magen Avraham explains the Gemara permitting the use of an elephant for a sukkah wall. See also Maharatz Chiyot to Shabbat 21; Shu”t Chatam Sofer OC 39; and Sdei Chemed, Asifat Dinim, Ma’arechet Chanukah, section 14, pp. 49-65. Drawing on a Tosafot in Sotah 17b, the Beit Yitzchak (YD 145) argues that even the Magen Avraham would permit the use of non-kosher oils and waxes for use on Chanukah and Shabbat. There the Tosafot permit writing the sotah text on non-kosher parchment since it will be destroyed. Similarly, the candles used for Chanukah and Shabbat are destroyed.
6. The Talmud (Yoma 76b) further derives (from Daniel 1:3 and 10:12) that the prohibition of anointing is considered “inui” (suffering).
7. This issue might also apply to bubble bath. To keep the bubbles from popping while floating on the hot water, hydrolyzed animal collagen is added. This is created from the tendons of presumably non-kosher animals.
8. Rambam (see Kesef Mishnah to Hilchot Shvisat Asur, chap. 1) maintains that the concept of sichah keshtiyah, at least in regard to Yom Kippur, is Biblical in nature.
9. He notes that the Levush points out that there is a specific aversion to using products made from pig products, although this may be more of a simple revulsion than a halachic mandate.
10. Rav Yitzchak Ratzaby [contemporary Yemenite posek; Shulchan Aruch Hamekutzar, YD 1 (5760), 218] similarly rules that one may lather oneself with soap made of forbidden tallow because it is ill tasting, and that is the prevalent custom. In support, he cites Beit Yosef YD 117; Kaf Hachaim OC 326:45; ibid, YD 117:17; Biur Halachah to OC 326:10; Shu”t Tzemach Tzedek 91; Yad Ephraim to YD 95; Yechave Da’at 4:43.
11. It seems that he is not agreeing with the Aruch haShulchan that everyone would permit today’s soaps. Rather, he is stating that the custom is to accept the lenient position of the Tosafot.
12. On this topic see also Rav Elchanan Peretz, “Din Sichah Beshemen Issur” (Hebrew), Mehadrin 7 (Adar 2, 5755), 76-86. See also Torah Loda’at, 3:47 and 8:24 for some of these sources, as well as other laws regarding anointing on Shabbat and Yom Kippur. Additionally, the Torah Loda’at discusses other laws pertaining to soap, such as non-kosher soap falling into food, chametzdik soap on Pesach and the use of soap on Shabbat.
13. See Peretz, ibid. Ironically, while there is good reason to use kosher soap, even though soap is never put in one’s mouth, it may not be necessary to use kosher toothpaste. That is indeed the opinion of Shearim Metzuyanim Behalachah (46:7; Braun on Kitzur Shulchan Aruch). Rav Shmuel David Munk (Peat Sudchah, YD 84 (5749), 160;) vehemently disagrees. He argues that tasting a prohibited item with one’s tongue is at least a rabbinic prohibition. This applies even if the item is not swallowed. He asserts that most people are careful to use kosher toothpaste.
14. An interesting, additional issue involving soap can arise in Israel. Cottonseed oil is used to manufacture soap, and during and after the shemittah year, there is a fear that cottonseed oils having kedushat shvi’it may be mixed into the soap. Most people are lenient in this regard and do not stock up on soap before seventh-year cotton oil hits the market. See Rav Moshe Sternbuch, Shemittah Kehilchata 3:18 and Rabbi Dovid Marchant,
Reprinted from JEWISH ACTION Magazine, Winter 5764/2003 issue