President Trump’s selection of Dr. Casey Means, a Stanford-educated wellness specialist and book author, as his next surgeon general provoked a vigorous debate on Thursday over whether someone who is not a practicing physician should serve as the nation’s top doctor.

Laura Loomer, the far-right activist who holds so much sway with Mr. Trump that more than a half-dozen national security officials were fired on her advice, emerged as a vocal critic. So did Nicole Shanahan, Health Secretary Robert F. Kennedy Jr.’s former running mate, who called the choice “very strange.”

Dr. Means, a close ally of Mr. Kennedy’s, trained as a surgeon in otolaryngology — ear, nose and throat medicine — but left her surgical residency after four years to practice “functional medicine,” which focuses on the root causes of disease. She has said she was disillusioned with the medical establishment, and had concluded it was failing patients.

Ms. Loomer ridiculed Dr. Means on social media Thursday, calling her a “total crack pot” who “USES SHROOMS AS ‘PLANT MEDICINE’ AND TALKS TO TREES!” and “DOESN’T EVEN HAVE AN ACTIVE MEDICAL LICENSE.” As evidence, Ms. Loomer posted excerpts of Dr. Means’s weekly newsletter, in which Dr. Means muses on recipes, product recommendations and other topics, including “mindset shifts and habits.”

Mr. Kennedy pushed back hard with a lengthy post on X, calling the criticism “absurd,” while Mr. Trump told reporters that he did not know Dr. Means, but picked her because “Bobby thought she was fantastic.”

Dr. Means’s brother, Calley Means, a White House adviser on health issues and former food industry lobbyist, defended his sister in an interview.