Very little is outside the realm of medical possibility. That’s different, though, from asking whether an idea is at all likely or worth investing time and resources investigating or trusting one’s health to. Trial and error and accumulated anecdote are notoriously poor ways of finding the truth about the physical world, and the list of useless or outright harmful therapies justified in this way by thousands or millions of people, often for centuries at a time, is long indeed. If even a portion of acupuncture claims are true, it becomes very difficult to explain why the practice did not succeed in achieving any noticeable improvement in the health or average lifespan of people where it was used. Certainly, the advent of scientific medicine has unequivocally had such an impact, and it seems appropriate that when we talk about efficacy, that is the standard we ought to use.
No doubt acupuncture does something, and I think the evidence is strong that it has its effects primarily by influencing the brain through belief, expectation, and other “psychological” mechanisms. The holdover of dualistic language we’re stuck with makes these effects seem somehow less “real” that if the effects were found to be in nerves in the arm or something, but of course beleifs, expectations, emotions, perceptions of symptoms, and all the mechanisms of the placebo effect are just as real as any health process, and they take place in a physical organ, namely the brain. We just don’t udnerstand the relationship between the organ and its output as well as we do for, say, the heart or the liver, so we tend to put psychogenic symptoms and therapies in a vaguely defined different category than other medical symptoms and treatment responses. And certainly they can be a lot harder to study since the act of studying them is itself a variable that can influence them in hard-to-predict ways.
But this is not justification for exempting acupuncture for the standards of evidence we apply to, say, antibiotics or pacemakers. I see no reason to treat acupuncture any differently than any conventional medical intervention in terms of investigating and judging its worth. And the recourse to “ancient wisdom” or anecdote as evidence only arises becase it cannot hold its own in terms of the usual, and I think superior, standards of scientific medicine.