Musar for Shabbat 134:15

Apart from the mission assigned to each herb individually, each herb performs other activities when it is merged with other species. When such mixing of species occurs there are aspects which are forbidden to man, because G–d is aware that the use by man of such mixtures will ultimately be harmful to him. For this reason, הלכה says that if a mixture of herbs is proven to possess therapeutic qualities, the use of it is not forbidden as something that is considered pagan practice (Shabbat 67). This means that though the pagans who deny G–d use this substance as a form of witchcraft, Jews are not therefore automatically forbidden therapeutic use of it. Matters forbidden under the heading of מפני דרכי אמורי, are only forbidden because of the ultimate harm this practice causes to its user. We need to appreciate also that there is another reason why such mixtures are forbidden, and that is because the power of the mixture is strong enough to neutralize, at least temporarily, the celestial agents directing and supervising their "charge" of each separate respective species which now have been combined.