We are not allowed to cut ourselves – not in service of an idol and not even in grief over a loved one’s death (see Leviticus 19:28). We see that this was an idolatrous practice in I Kings chapter 18, where the priests of the Baal cut themselves as part of their service. While only cutting for the dead or for an idol would incur a court-imposed penalty, cutting oneself is equally prohibited for any other reason, or for no reason at all.
There are numerous reasons not to cut one’s self. If done for an idol, it violates the prohibition against idolatry. If done in grief for the dead, it is a rebellion of sorts against God’s judgment. In any case, it violates the Biblical injunction to protect our lives and our health (Deuteronomy 4:15) and, like tattooing, is a flagrant abuse of the bodies God has entrusted to our care.
There are those, particularly teenage girls, who suffer from a psychological compulsion to cut themselves. Religious objections aside, such behavior is dangerous and, believe it or not, contagious to the peer group. Cutting should be taken seriously by parents, who should be aware that simply ordering a cutter to stop is insufficient to ensure behavior modification.
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Makkos (20b-21a). It is codified in the Shulchan Aruch in Yoreh Deah 180. This mitzvah is #45 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #28 of the 194 negative mitzvos that can be observed today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.