Shu”t Tzafnas Paneach Chelek 1 S 14: “The remedies mentioned in the Gemara (though nowadays not applicable) are a part of the Torah Shebal peh.”
As opposed to his medical books (you could read them in the bathroom), the specific instructions in Hilchos Deios Ch. 4 come from the Gemara (migdal oz, malbim, hagahos bnei binyomin, tzaf”p).
(But thats besides the point because even if not it still is bigeder Torah that you can’t read in the bathroom. See Lechem Mishna Hilchos Talmud Torah 4:1 and the end of the Rambam’s hakdomo.)