Shulchan Aruch (OC 230:4, as explained by Magen Avraham ibid. :6) paskens that when one undergoes a medical procedure he should say the following prayer:
יהי רצון מלפניך ה׳ אלקי שיהא עסק זה לי לרפואה כי רופא חנם אתה
I would translate the first part of the Tefillah as: May it be pleasing before You, HaShem, my G-d, that this undertaking be for me for health.
However, that last phrase, כי רופא חנם אתה, seems odd to me. חנם usually means "free, without payment" (as in ויצאה חנם אין כסף, she will leave for free, without payment, in Shemos 21:11). Thus, the phrase would seem to be translated as, "for You are a Doctor Who does not charge."
Why do we make this point here? HaShem, allow this treatment that I'm spending thousands on to work, because You work for free? I could understand if the Tefillah was רופא חסד, a kind doctor, or רופא צדיק, a righteous doctor, or a רופא נאמן, a trustworthy doctor. But why רופא חנם?