Red flags are clinical signs and symptoms that may indicate serious underlying conditions with potentially poor prognosis if missed. Originally established in orthopedic and emergency medicine guidelines, the concept is highly relevant to acupuncture practice, where practitioners often encounter patients without prior medical evaluation. This review summarizes the key red flags discussed in an educational lecture at the 74th Annual Meeting of the Japan Society of Acupuncture and Moxibustion. Common warning patterns include cold sweats, sudden onset of symptoms, and acute events described as obstruction, rupture, dissection, or torsion. Vital signs-such as consciousness level, blood pressure, pulse, respiratory rate, body temperature, and oxygen saturation-are emphasized as essential non-invasive tools for rapid assessment. Symptom-specific red flags are outlined for headache (SNOOP criteria), shoulder pain (cardiac, hepatobiliary, pulmonary causes), low back pain (TUNAFISH criteria), and pregnancy (fetal movement decrease, rupture of membranes, vaginal bleeding). Based on the Acupuncture Safety Guidelines in Japan, immediate cessation of treatment and referral to appropriate medical facilities are recommended when red flags are present. Early identification and appropriate triage by acupuncturists can significantly contribute to patient safety and improve outcomes through timely medical intervention and interprofessional collaboration.