ייִדישער מחשבֿה אויף מלכים ב 2:19

It is even possible that He will heal him through something that is normally very harmful, as you know from the story of Elisha and the bad water, that he healed their damaging properties with salt (Melachim II 2:19), and similarly "And G-d showed him a tree and he tossed it into the waters [and the waters became sweet]" (Shmos 15:25), And the midrash tanchuma there explains that this was a bitter, oleander tree. Another example, "let him smear crushed figs on the boils, and he will heal" (Yeshaya 38:21) (and figs normally damage even healthy flesh - PL). And you already know of what happened to the pious king Asa when he trusted in the doctors, and removed his trust in G-d regarding his illness, the sharp rebuke he received for this (i.e. because he did not pray to be healed). And the verse says: "For He brings pain and binds it; He wounds, and His hands heal." (Iyov 5:18).