Modern research approaches acupuncture along several routes: neurophysiology, particularly the modulation of pain transmission and the role of endogenous opioids; autonomic and endocrine effects; immunological changes; and functional imaging of brain responses to needling.
Clinical evidence is uneven. It is strongest for chronic pain, tension headache, migraine and chemotherapy-induced nausea, and weakest for conditions where the underlying pathology is structural. Systematic reviews consistently find that acupuncture performs better than no treatment and that the difference from sham is smaller and less consistent. The honest summary is that acupuncture has measurable physiological effects and a modest but real clinical effect in a limited range of conditions.