By: Rabbi Ari Enkin
Although it might sound somewhat funny, making an effort to use only exclusively kosher soap for washing one’s body is not without merit. From ancient times right to this very day, soap is frequently manufactured from animal fats and other non-kosher sources. Even the famous Ivory brand of shower soap which claims to be more than 99% pure may contain beef and pork substances.
While it is true that with the exception of actual consumption, one is permitted to derive most other forms of benefit from non kosher items, soap may be an exception to this rule.  Indeed, it is permitted to do business with non-kosher items, play football with pig skin footballs, and even use candles made from non-kosher sources for the Shabbat or Chanukah lights. 
To better understand why soap is an exception to this rule, it is necessary to recall the restrictions of Yom Kippur. The distinctive restrictions of Yom Kippur consist of not eating or drinking, washing, anointing oneself, wearing leather shoes, and engaging in marital relations.  Among the reasons that anointing is forbidden on Yom Kippur is due to its relationship with drinking, a concept known as “Sicha K’shtiya” in halachic literature. It is taught that smearing [oils on one’s skin] and drinking are two comparable activities. As such, many authorities have broadened the Sicha K’shtiya concept to assert that the use of non-kosher soaps is comparable to drinking non-kosher beverages and should be likewise forbidden.
Those who dismiss the concerns raised regarding the use of non-kosher soap base themselves upon the view that the principle of Sicha K’shtiya is only relevant to pure oils, not to other items smeared upon one’s skin, such as animal fats. Another lenient consideration has it that since Sicha K’shtiya is primarily a rabbinical enactment, there is no reason to extend it from its Yom Kippur related origins. Yet other authorities dismiss any concerns on using non-kosher soaps because no one would ever consider eating them due to their awful, inedible taste.
Although the consensus of contemporary halachic authorities is to permit the use of any soap, there are eminent authorities who strongly urge the use of only certified kosher soaps. Furthermore, although most authorities dismiss the concerns regarding the use of non-kosher soap for washing one’s body, many individuals are nevertheless careful to use only kosher soap for cleaning one’s dishes and other eating utensils.
 This post is based on “What’s the truth…about kosher soap” by Rabbi Ari Zivotofsky, cited at: http://www.ou.org/publications/ja/5764/5764winter/LEGALEAS.PDF
 Pesachim 21-23, Rambam Ma’achalot Assurot 8:15
 Beit Shlomo O.C. 108,109
 Yoma 73b
 Shabbat 86a, Yoma 76b, Rambam Teruma 11:1, Rambam Ma’aser Sheni 3:10, 7:17
 Taz Y.D. 117:4
 Tosfot Nidda 32a
 Aruch Hashulchan Y.D. 117:29
 Aruch Hashulchan Y.D. 117:29, Biur Halacha O.C. 326, Igrot Moshe O.C. 3:62, Be’er Moshe 3:87, Yechave Da’at 4:43
 Kaf Hachaim Y.D. 117:15, Taz Y.D. 117:4, Vilna Gaon cited in Biur Halacha O.C. 326
 Igrot Moshe Y.D. 2:30