Q. What is the source of the red string or roite bendl that some people wear on their wrist?
Is it permitted or is it Darkei Haemori?

A. Poskim disagree as to the propriety of this popular segula. Orchoseicha Lamdeni (140) prohibits wearing it on the hand. Hakoton Vehilchoisov (81) quotes a reponsa from Hagrach Kanievsky Shlit”a, ruling that one should abstain from placing such string on a minor to protect him from ayn horah (evil eye), since it may be similar to what the Tosefta (Shabbos 8: 4) considers to be one of the “ways of the Emorites.” However, he mentions that the above may be restricted only to tying the red string on the finger and may not apply to other bodily parts. He quotes such a tradition from the Yerushalaim’s elders, namely to go around Kever Rochel seven times with a red string, before placing it around the neck of the ill. Yechave Da’as (6: 50) asserts that what is not recorded in the Talmud itself, will not be prohibited for being “the ways of the Emorites.”
Be’er Moishe (8: 36) maintains that tying the red string on the hand or the carriage of a child, is a tradition that many used to keep. It is included in the minhagim of our elderly mothers that the Rashbo advises not to belittle, since they have their roots on holy high-ground; even if we do not understand now their reason. He quotes similar opinions from Kol Bo on mila (p.129) who relates that the Yismach Moishe would advocate the use of the red string for perilous pregnancies. Vayizbor Yosef (115, p. 409) permits the use of a red string chain with a ruby pendant, as long as the intention of its use is for heaven’s sake. Some maintain that the origin of the red string remotes to Kolev’s and Pinchas advise to Rochov, to save herself by keeping the “Tikvas Hashoni” in her window.
Others claim that it goes back to even earlier times: to the birth of Zerach, son of Yehuda and Tamar. The posuk (Bereshis 38: 28) reads; And it came about when she gave birth, that he (the infant) stretched out his hand. So the midwife took and bound a crimson thread on his hand, saying, “This one came out first.”
Horav Shlomo Miller’s Shlit’a opinion is that one should maintain the minhagim and traditions of one’s family.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a