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What is chronic migraine, and how are the days counted?

Chronic migraine is defined by how often headaches happen over months. That puts the count of days, and the record behind it, at the center of a neurology visit.

Arun Verma, co-founder of Triangle · 2026-10-19

This is not clinical evidence and not medical advice. It does not replace your care team. Discuss anything here with your clinician before you act on it.

What is chronic migraine?

Chronic migraine is headache on 15 or more days a month for more than three months, with migraine features on at least 8 of those days. That is the wording of ICHD-3, the International Headache Society's classification of headache disorders, published in Cephalalgia in 2018. NINDS puts it in similar terms: headaches from migraine on at least 15 days of the month for more than three months, with or without aura. Migraine itself is a health condition that is more than a bad headache, and its symptoms often include headaches that keep coming back. What sets chronic migraine apart is how often the headaches happen. About 2% of people live with chronic migraine, according to Brain & Life, the American Academy of Neurology's site for patients.

How is chronic migraine different from episodic migraine?

The difference is frequency. An American Headache Society (AHS) patient guide says whether migraine is chronic or episodic is defined by how often headache episodes happen, and puts episodic migraine at fewer than 15 headaches a month. ICHD-3 explains why chronic migraine gets its own category: when headaches are this frequent or continuous, the individual episodes can't be told apart, and the headache's character may change from day to day or even within the same day. The line can be crossed. The same AHS guide says studies estimate that 2.5% of people with episodic migraine move to chronic migraine every year, and that the most obvious sign is more frequent attacks.

Do all 15 headache days have to be migraine?

No. Only 8 of them need migraine features. ICHD-3 counts every headache day toward the 15, whether it is migraine-like or tension-type-like, and counts days with and without aura alike. Headaches caused by another condition, which ICHD-3 calls secondary headaches, are left out of the count. There is a history requirement too. The criteria say chronic migraine occurs in someone who has had at least five migraine attacks, with or without aura, and that the picture is not better explained by another headache diagnosis.

What makes a headache day a migraine day?

A set of features ICHD-3 spells out for migraine. The headache has at least two of four traits: it is on one side, it pulses, the pain is moderate or severe, and routine activity such as walking or climbing stairs makes it worse or makes you avoid it. During the headache there is also nausea or vomiting, or sensitivity to both light and sound. A full migraine attack in adults lasts 4 to 72 hours when untreated or unsuccessfully treated, in ICHD-3's criteria for migraine without aura. For the chronic migraine count, the question for each of the 8 days is whether that day's headache meets those feature criteria, or the matching criteria for migraine with aura.

How is chronic migraine diagnosed?

From your history and your description of the headaches, not from a scan. There is no single test for migraine, Brain & Life says; a doctor, often a neurologist, makes the diagnosis from your medical history and symptoms. NINDS says a doctor may ask when headaches start, how long they last, what they feel like, what triggers them and whether nausea or vision changes come with them. An MRI or CT scan isn't usually used to diagnose migraine. NINDS says a doctor might order one to look for another condition that could cause the headaches. How the headaches began matters as well. ICHD-3 notes that chronic migraine evolves over time from episodic migraine, while a headache that is daily and unremitting from within 24 hours of its first onset is classed as new daily persistent headache. When the onset isn't remembered or is uncertain, the classification calls it chronic migraine.

Why do doctors ask how often I take something for a headache?

Because medication-overuse headache is a recognized diagnosis, and it can look like chronic migraine. ICHD-3 describes it as headache on 15 or more days a month in someone with an existing headache disorder, developing as a consequence of regular overuse of acute or symptomatic headache medication for more than three months. Older names for it include rebound headache. ICHD-3 notes that the most common cause of symptoms suggestive of chronic migraine is medication overuse, and that someone who meets the criteria for both should be given both diagnoses. An AHS patient guide calls medication-overuse headache a secondary headache disorder, most often seen in people living with migraine, and says that if you find yourself in that situation, it's not your fault. A fair question for your neurologist is how they are counting the days you took something for a headache.

Why does a headache diary matter for chronic migraine?

Because the diagnosis rests on a count of days over months. ICHD-3 says describing frequently recurring headache generally requires a diary of pain and related symptoms, kept day by day for at least one month. An AHS patient guide describes a daily diary of how many headaches you have each month as a way to prepare for a headache specialist. NINDS lists what to note after each headache in a headache journal: the time of day, how intense it was and how long it lasted, any sensitivity to light, smells or sound, what you were doing right before it started, and any medicines you took, prescription or over the counter. ICHD-3 counts tension-type-like days toward the 15 alongside migraine-like ones, so milder days belong in the same count.

Why does a record of past medicines help a new neurologist?

Because what you have already tried, and how often, is part of the history a neurologist reads. NINDS says not every preventive migraine treatment works for every person, and that some people need to try several options before finding one that works well for them. ICHD-3 even has a holding code for people who are clearly overusing several headache medicines but cannot give an adequate account of their names or quantities, used until better information is available, and it says that in almost all cases this means diary follow-up. A fair question is which of the medicines you remember trying actually appear in the records the neurologist has, and with what dates.

What could I ask a neurologist about a chronic migraine diagnosis?

You could ask which criteria the diagnosis rests on, how many headache days a month your records show and over what span, and how many of those days had migraine features. Other fair questions are whether medication-overuse headache was considered alongside it, since ICHD-3 allows both diagnoses at once, and what your history says about how the headaches first began. What your own count means for you is a question for the people who know your history.

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