mercurialist

from The Century Dictionary.

• noun One who is under the influence of the planet Mercury, or one resembling the god Mercury in fickleness of character.
• noun A physician much given to the use of mercury in the treatment of disease.
• noun A scholar; a rhetorician.

from the GNU version of the Collaborative International Dictionary of English.

• noun One under the influence of Mercury; one resembling Mercury in character.
• noun (Med.) A physician who uses much mercury, in any of its forms, in his practice.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun archaic, medicine A doctor who uses mercury or supports its use in medicine.