A common treatment in the times of the mishnayos for someone bitten by a rabid dog was for him to eat from the dog's liver. Although in cases of Pikuach Nefesh the issur of eating treif does not apply, the Chachamim argue with Rav Masya ben Charash (Yuma 8:6) and say that we do not allow someone to eat it when bitten.
The Rambam says (Peirush HaMishnayos) that we pasken like the Chachomim that he cannot eat it. The reason he says, is because we cannot be oveir on the torah unless it is a natural cure based on medicinal properties, and not a cure based on Segulos. The Nishmas Avrohom brings the Chida who says that it seems from the words of the Rambam that even if the Segula is proven to work and there is no doubt about its efficacy, still we cannot violate an issur.
Rav Shlomo Kluger says that the Shulchan Aruch (OC 301:25) that allows a person to go into Reshus HaRabim wearing a Kmei'a (that is proven to work) on Shabbos is not a contradiction to this Rambam. The reason why that is permitted is because to the person wearing it, whether it truly works or not, it is no worse than a piece of jewelry which is permissible to wear on Shabbos, and not because of refuah.
Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.