One may also not try to work magic by casting spells. This refers to muttering words and then telling people that the incantation effected some change in reality. This includes speaking charms to various dangerous beasts so that they will not harm him or saying a spell over a wound to alleviate pain.
The obvious question, or perhaps a cynical one, is “What's the difference between that and saying Tehillim (Psalms) for sick people?” The difference is that Psalms are meant to inspire us. They are supposed to give us strength and help us to turn to God in our hour of need. That's a far cry from, “Abracadabra! Hocus pocus! Okay, I changed nature and now you're cured. (That'll be fifty bucks, please.)” If we were to use words of Torah in such a fashion, it would be equally prohibited.
The reason underlying this mitzvah is what we have already said about the various, so-called “mystic arts.” Such chicanery misleads people and causes them to stray from God, Who is the Real Thing.
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Sanhedrin (pages 65a-b) and is codified in the Shulchan Aruch in Yoreh Deah 179. This mitzvah is #35 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #169 of the 194 negative mitzvos that can be observed today in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.