**R51 — Headache** is a non-specific symptom code used when the underlying cause is not yet established. It refers to pain in any part of the head, including tension-type, secondary, or undifferentiated headache.
Clinicians select R51 in primary-care and emergency settings before a definitive diagnosis (e.g. migraine [G43], cluster headache [G44.0], or sinusitis-related headache [J01]) can be made.
Belongs to **Chapter R** — Symptoms, signs and abnormal clinical and laboratory findings, used when no more specific diagnostic code applies.
