Someone asked me not long ago whether it is permitted to smear on one's skin items that are not kosher. This person in particular is very health conscious and is trying to figure out creative ways of absorbing certain items that they believe to be healthy into their body without violating any issurim. The question is, when the gemara makes the claim that סיכה כשתיה, how seriously do we take that? Do we view smearing oneself with something to be tantamount to drinking it? There is no question that in the context of Yom Kippur, the gemara clearly does. In the context of one who is forbidden to eat teruma using it to on their body is also explicit in the gemara Nidah 32a that it is prohibited. It is also obvious that issurei hana'ah one cannot use at all, therefore one could certainly not use Orlah which is assur b'hana'ah (as the gemara says in Pesachim 25b). The question is whether one can use other forms of forbidden foods to smear themselves with.
Tosafos 77a d.h. d'nan writes in the name of Rabbeinu Tam that aside from eating and drinking all the עינויים of Yom Kippur are only d'rabonon. Tosafos continues that the Rabbonon were only gozer that סיכה כשתיה by Yom Kippur and for Teruman, but nowhere else. Clearly Rabbeinu Tam holds that it is perfectly permissible to use all forms of ma'achalos ha'asuros and rub it on one's skin, so long as they don't ingest it. However, it seems from Tosafos that this approach is predicated on the assumption that the concept of סיכה כשתיה even where it does apply (Yom Kippur and Teruma) is only d'rabonon. However, if we assume that the עינויים of Yom Kippur are d'oraysa, as the Ran quotes from the Rambam, the concept of סיכה כשתיה must be d'oraysa at least in the context of Yom Kippur. Similarly, the Rambam (Terumos 10,2) implies that eating, drinking and anointing are all forbidden forms of using teruma (for one who may not use teruma), and that they are all the same, meaning that they are d'oraysa.
The question becomes, according to the Rambam who holds that the concept of סיכה כשתיה is d'oraysa, how does it apply to other issurim aside from Yom Kippur and Teruma. Do we assume that it is only d'oraysa in those two places, or are we to learn from there that it is always d'oraysa? And even if it is only d'oraysa in those two places, is it at least an issur d'rabonon elsewhere or is it completely mutar?
Even within the opinion of the Rambam that סיכה כשתיה is d'oraysa by Terumah and Yom Kippur, the Machane Ephraim (hearos at end of sefer - pg. מא in the old editions d.h. עוד בדין) writes that only a sicha done for the purpose of enjoyment would be d'oraysa, not if done for medicinal purposes. Therefore, the proof of Tosafos from Yoma 77b about one who has head injuries being able to do sicha is not a question on the Rambam because it is not for ta'anug, therefore only d'rabonon, and for refuah they weren't go'zer.
The Taz (Y.D. 117:4) cites the Beis Yosef quoting the Orchos Chaim that one may not anoint themselves with pig fat דסיכה בכלל שתיה היא. The Taz rejects that based on our Tosafos, and from the Rashba who permits anointing with pig fat, even when there is no sakana. However, The Taz concludes by citing the Issur V'heter that it is only permitted if there is some level of tza'ar, one who is perfectly healthy and is only using it as a moisturizer or for תענוג בעלמא, it may not be done. The Taz comments - וכן עיקר, implying that it can only be done to alleviate a tza'ar but cannot be done for just simple enjoyment.
It is very difficult to understand the hagdara of the Issur V'heter and Taz. If we say סיכה כשתיה is should be a regular issur, whether d'oraysa or d'rabonon, and should not be mutar to avoid tza'ar. The fact that it is mutar to avoid tza'ar implies that it is not a real issur, so why is it assur to use for pleasure?
The Nekudos Hakesef also quotes the same sources and seems to hold that Tosafos clearly permits it entirely, even for תענוג בעלמא, which is also the p'sak of the Bach. However, the Nekudas Hakesef comments that he saw those who are machmir not to wash with non-kosher soap, "and that is correct". He then continues to try and justify a minhag of not using pig fat in particular, while still using other forms of non-kosher fats.
In short, both the Taz and Nekudas Hakesef seem to conclude that it is technically permitted to use non-kosher items on one's skin, but one should preferably abstain from doing so for the sake of enjoyment. One should only use it if they NEED it. Similarly, the Aruch Hashulchan (29) justifies the practice of using soap from non-kosher items even for no particular need since it is already pagum, which implies that he agrees that otherwise it should not be used lichatchila. The Hagdara of the issur still needs explanation.
Perhaps the Issur V'heter holds that the distinction between תענוג and to alleviate צער is not a practical distinction of how necessary it is, Rather, he holds that the real issur of סיכה כשתיה is only when done for תענוג, as the Machaneh Ephraim says explicitly, but when done to avoid tza'ar, that does not qualify as ta'anug. Therefore, even other issurim outside of Y.K. and Teruma, we only permit it when it is to help alleviate tza'ar, but if it is done for ta'anug the Issur V'Heter is concerned for the opinion of the Rambam that it is d'oraysa on Y.K. and for Teruma, and therefore may apply to other issurim as well.
Regarding the issue of anointing oneself using items that are meant to be ingested, there is definitely not going to be an issur d'oraysa because the Rambam (Terumos 10:2 and 11:1) implies that even for Teruma we only say סיכה כשתיה if it is meant to be used in that way. If it is meant to be ingested, even the Rambam would hold that at worst it would be a d'rabonon. Being that we don't know what the Rambam would hold for other issurim outside of Terumah and Yom Kippur, and Tosafos is matir explicitly, one who feels that they need this item for their health can certainly be lenient to use it.