Just as one may not shear an animal’s wool, so too one may not remove anything growing from the human body, such as hairs, nails, loose skin, and warts. One who removes them in the usual fashion transgresses a Torah prohibition, while one who removes them with a shinui transgresses a rabbinic prohibition.
Therefore, one who cuts his nails using a nail scissors or clippers transgresses a Torah prohibition, while one who pulls them off with his teeth transgresses a rabbinic prohibition (MT 9:8). One should make every effort to stop biting his nails entirely; apart from being impolite, a habitual nail biter is likely to bite his nails on Shabbat as well, thus violating Shabbat.
Similarly, one may not remove warts. If the wart is wet, cutting it with an instrument is a Torah prohibition, while removing it with a shinui, via teeth or hands, is a rabbinic transgression. If the wart is dried out and liable to fall off on its own, then even removing it with an instrument is only a rabbinic transgression (ibid.; SA 340:2; MB ad loc. 6).
One may not scrape the body to remove skin that has begun to peel. One may not even use one’s teeth to remove the skin from his chapped lips. However, one may remove dandruff, since it is only very loosely attached to the skin, and it falls out readily.
If a nail was torn off most of the way and is now painful, it may be removed using one’s hands or one’s teeth. Since most of it has already been torn off, it is viewed as if it has already fallen off, and the prohibition on finishing the job is only rabbinic. The Sages permit removing the nail with a shinui if it is causing pain (Shabbat 94b; SA 328:31; Ĥayei Adam 21:4).
One may not scratch a cut in a way that will make it bleed, because doing so constitutes Ĥovel (see below 20:9). Even though one who scratches is not interested in drawing blood, it is still rabbinically prohibited (MB 316:30). Similarly, one may not brush his teeth if it is almost certain that his gums will bleed. One also may not knowingly suck the blood out of his gums after brushing them to the point that they bleed, as this constitutes Ĥovel (MB 328:147; section 7 below).
One may remove a splinter lodged in a person’s flesh as long as one is careful not to draw blood. If the splinter is painful, one may remove it even it will certainly cause bleeding. This is because drawing blood in this situation is rabbinically prohibited, and the Sages did not extend their prohibition to cases that entail such pain (MB 328:88).
One may remove a scab from a wound that has dried and will not bleed upon the scab’s removal. This does not qualify as Gozez since scabs do not grow from the body, but rather are just dried blood from a wound (SA 328:22).