Gemara

61a (Mishnah): One may not go out with a Kemi'a (amulet) that is not [established to heal, i.e.] from an expert.

65a (Mishnah): A woman may go out with a coin on a wound on the sole of the foot.

(Abaye): A coin helps cure it because is hard, moisture exudes from it, and due to the image jutting out of it.

67a (Mishnah - R. Meir): One may go out (on Shabbos) with a locust egg, the tooth of a fox, or a nail from a crucifix for the sake of a cure;

Chachamim forbid these even during the week due to Darchei ha'Emori (following Nochri superstitions).

One hangs the locust egg in the ear for an ear ache. A tooth from a live fox helps one who sleeps too much, and a tooth from a dead fox helps one to sleep. A nail from a crucifix helps for swelling (due to a blow from iron).

(Abaye and Rava): Anything done to heal is not (forbidden due to) Darchei ha'Emori. If it does not (have a logical or medical reason why it should) heal, it is Darchei ha'Emori.

128b (Beraisa): If one who gave birth needs a lamp, others may light a lamp for her [on Shabbos].

This is even if she is blind. Even though she cannot see, it calms her to know that it is light and others can see her needs.

Yoma 83a (Mishnah): If a rabid dog bit a person, we may not feed to him from its liver;

R. Masya ben Charash permits.

Sanhedrin 101a (Beraisa): We may whisper charms to snakes and scorpions.

Rishonim

Rambam (Perush ha'Mishnayos Yoma 8:6): The Halachah does not follow R. Masya, who permits feeding liver of a rabid dog on Yom Kipur to one whom it bit, for this works only through Segulah. Chachamim permit transgressing a Mitzvah only for a Refu'ah that is clear through logic and experience, but not for a Segulah.

Rashi (83a DH Ein) and Bartenura (Yoma 8:4): Even though doctors regularly use this cure, it is not a total cure to permit eating from a Tamei animal. R. Masya holds that it is a proper cure.

Rashi (101a DH Locheshin): We may whisper charms to snakes and scorpions so they will not damage. This is not considered trapping.

Hagahos Maimoniyos (Hilchos Shabbos 21:4): One may say incantations on Shabbos, like it says in Sanhedrin. This is not included in the decree against Refu'ah. Once, R. Simchah's eye hurt, and he told people to whisper on Shabbos.

Rashba (1:825): Sifrei Refu'os of Rishonim say that the form of a lion on a silver plate helps for a certain sickness. I say that there is no concern for the Isur of making images of Hash-m's servants above, even if a Yisrael made them protruding. The Isur is to make all four images on the Chariot (man, lion, ox, Nesher (eagle or vulture)). This is why a Drashah was needed to forbid making man's image [alone]. Darchei ha'Emori does not apply to any cure known to doctors. We do not forbid what the Gemara did not forbid due to Darchei ha'Emori, for we do not know the Segulos. We cannot judge from known natural laws. There are some Segulos that no scientists understand, e.g. Kemi'os of herbs, cooling of a green rock called Eshtopasi, and incantations that Chachamim permitted. They even commanded to say them due to danger! They said that certain forms on coins heal. The Ramban used to make the form of a lion [to heal].

Ran (Shabbos 30b DH Kol): Even if we are unsure whether or not an incantation cures, it is permitted due to Safek. We forbid only what is known not to work. The Rashba says that Tosfos connotes like this. However, R. Yonah (Sha'arei Teshuvah 3:104) was concerned for Darchei ha'Emori for any Kami'a not known to cure.

Poskim

Shulchan Aruch (OC 301:27): One may go out on a weekday or Shabbos with a locust egg, the tooth of a fox, or a nail from a crucifix for the sake of a cure. This is not Darchei ha'Emori. The same applies to anything for Refu'ah. If one does an action and it is not Nikar (recognized) that it is for Refu'ah, it is forbidden due to Darchei ha'Emori. Every incantation is permitted. Chachamim forbade only those that were tested and found not to work.

Mishnah Berurah (105): If people who know that say that something heals through Segulah, it is as if it is Nikar that it heals.

Mishnah Berurah (106): Even if we do not know that an incantation heals, since we know that some heal, we say that perhaps also this one heals. It is as if it is somewhat Nikar that it heals.

Shulchan Aruch (ibid.): Some are concerned for Darchei ha'Emori for every Kami'a that is not expert.

Gra (DH Yesh): This is difficult, for the Mishnah forbids a Kami'a that is not expert [on Shabbos]. This implies that it is permitted on a weekday!

Levushei Serad: This opinion explains that the Mishnah teaches that there is an Isur Shabbos for those not known to heal, in addition to Darchei ha'Emori.

Mishnah Berurah (107): Levushei Serad struggled to answer. If there is concern for danger, it seems that all permit.

Kaf ha'Chayim (161): The Radvaz and Birkei Yosef permit children to go out with Almogim (coral) for Ayin Ra'ah. Even though it is not established, they intend for a Tachshit.

Kaf ha'Chayim (164): A Ba'al Nefesh should be concerned for the stringent opinion if there is not such a great need.

Shulchan Aruch (328:45): One may whisper [charms] to snakes and scorpions on Shabbos so they will not harm him. This is not forbidden due to trapping.

Mishnah Berurah (143): Even though through the charm they cannot move and one could seize them, it is permitted because it is not natural trapping. When they are chasing a person, one may properly trap them. Here, they are not chasing him. It is a distant concern lest they damage. (Therefore, one may trap them only through incantations.)

Question: Reuven was established to periodically go insane. He must be locked up in iron chains. It lasts for a few days, and he totally recovers. Shimon said that if he eats a chicken that died by itself, he will not go crazy again. He saw that someone else like this was cured through this. Also Benos Yisrael testified that this cured insanity. May we feed the Nevelah to him?

Answer (Admas Kodesh 1 YD 6): If a woman gave birth on Shabbos, we may light a Ner, even if she is blind, for it calms her mind. The Rashba learned from here to a man who swore not to play monetary games, and went insane. People played in front of him, and he improved. We should permit his oath, due to Piku'ach Nefesh, like the Yoledes. However, the Rashba was unsure, for perhaps he merely seeks a Heter for his vow. Here, this is not a concern. We may break a wall locked in front of a child, lest he be afraid.

Admas Kodesh (ibod.): If a rabid dog bit a person, Chachamim and R. Masya ben Charash argue about whether one may feed to him from its liver. The Halachah follows Chachamim, who forbid. The Rambam explains that they hold that one may transgress Mitzvos for Piku'ach Nefesh only for natural cures, but not for irrational Segulos. However, the Rashba disagrees. He permits engraving the form of a lion on a silver or gold plate, like it says in Sefer Refu'os of Rishonim. He says that whatever the Gemara did not forbid due to Darchei ha'Emori, we do not forbid, for we do not know the Segulos. Also the Ramban made the form of a lion for this sickness. The Rashba (413) brought many proofs against the Rambam. We rely on him and his proofs and the Ramban. A Teshuvah of Hagahos Maimoniyos (Ma'achalos Asuros 14:2) permitted a Nochri to heal an epileptic, even though the cure included Sheretz ha'Of, as long as the cure was known. It is better to feed the patient Nevelah, and he will transgress a Lav, so he will heal and fulfill all Mitzvos.

Question: A certain man found a Segulah from the Rishonim for one who drank poison to write a certain Kami'a. It causes the person to vomit it out and recover. He saved several people this way. Once, it occurred on Shabbos. He wrote the Kami'a, and it saved her. Was this permitted?

Answer (Birkei Yosef 301:6): Seemingly, the Heter of Piku'ach Nefesh on Shabbos is only for natural cures. If a rabid dog bit a person, The Halachah follows Chachamim, who forbid feeding liver from a rabid dog. The Rambam explains that they permit transgressing Mitzvos for Piku'ach Nefesh only for natural cures. Even though it is established that this Segulah works, one may not transgress for it. Even though one may go out (in Reshus ha'Rabim) with an expert Kami'a or a locust egg, we did not permit a Torah Isur. This is not carrying the way people carry. The Radvaz (Leshonos ha'Rambam 153) explains that the Rambam holds that Segulos are not Refu'os at all. One may not do an Isur Torah even for a Safek Refu'ah. However, Admas Kodesh said that the Rashba and Ramban permit, and he relied on them. Also Pri ha'Aretz permits, and this was followed in practice.

Shiyurei Berachah (6): This refers to doing a proper Melachah. Surely one may say a Segulah, even if it will cause Melachah to be done. See Yafeh l'Lev 3:334.

Yabi'a Omer (1 OC 19:8,10): Hilchos Ketanos (2:98) says that one who kills through Hash-m's name or witchcraft is considered like one who did an action, like Temurah. Ma'ase Roke'ach (Hilchos Shabbos 24:7) says that he is liable for Melachah on Shabbos, as if he killed with a knife. This supports Geza Yishai (1:1), who forbids creating a person or animal through oaths using Kodesh names. This is not considered mere Gerama (causation). Also the Chida and Yehudah Ya'aleh rule like Hilchos Ketanos. Lev Chayim (2:188) permits holding up Kodesh names in front of a fire on Shabbos, to extinguish it. The Yerushalmi supports this. David asked from the Urim v'Tumim on Shabbos (Eruvin 45a), even though the letters stick out or join up. For something that works naturally, surely one's speech is considered an action.

Pischei Teshuvah (YD 179:7): Some people invert a Kli to cause a Ner Shabbos to burn well. Amudei Kesef says that there is no concern for burning, since he does not touch the Ner, but one should not do so [even on a weekday] due to concern for Darchei ha'Emori.