Chronic migraine is defined as ≥15 headache days per month. That number determines your diagnosis, your treatment options, and what your insurer will cover.
The line between episodic migraine and chronic migraine is a single number: 15. Fewer than 15 headache days per month puts you in the episodic category; 15 or more, sustained over three months, crosses the clinical threshold into chronic migraine. That distinction shapes your diagnosis, determines which preventive medications are available to you, and, critically, governs what your insurer will authorize.
The International Classification of Headache Disorders (ICHD) defines chronic migraine as 15 or more headache days per month for more than three consecutive months, with at least 8 of those days fulfilling criteria for migraine (either with or without aura, or responding to migraine-specific treatment). The remaining headache days may be tension-type or other headache; they still count toward the monthly total.
Episodic migraine occupies the space below that threshold: fewer than 15 headache days per month. The criteria do not further subdivide episodic migraine into named subtypes, but headache specialists routinely talk about two practical categories: low-frequency episodic and high-frequency episodic. Low-frequency is generally fewer than 8 migraine days per month. High-frequency episodic sits in the 8 to 14 day range, one attack away from chronic territory, and often managed accordingly.
One important note on terminology: "headache days" does not mean days when you feel a little off. It means any day with a headache lasting at least four hours, or any day when you took a headache-specific medication and the headache resolved before the four-hour mark. That specificity matters enormously when you are trying to document where you fall.
The episodic-to-chronic distinction reflects a genuinely different disease state, not just a bureaucratic cutoff. Chronic migraine is associated with greater disability, higher rates of comorbid anxiety and depression, more sleep disruption, and worse quality of life across nearly every measure. It also responds differently to treatment, including which preventive therapies have demonstrated efficacy and received FDA approval in that specific population.
Calling someone with 4 migraine days per month "chronic" would be clinically wrong and would expose them to treatments whose risk-benefit profile isn't calibrated for their disease burden. The reverse is equally problematic: misclassifying someone with 16 headache days per month as "episodic" leaves them without access to treatments that could significantly reduce their frequency. The definition exists to match treatment intensity to disease severity.
Patients in the high-frequency episodic range, 8 to 14 headache days per month, occupy a clinically important middle ground. Their diagnosis is technically episodic, but their disease burden approaches that of chronic migraine. Specialists often treat high-frequency episodic patients with the same preventive urgency applied to chronic migraine because the risk of transformation is highest in this group, and because some CGRP preventives have demonstrated benefit across both populations.
If you are in the 8 to 14 day range, the specific count matters. A patient at 12 days per month and a patient at 14 days per month are both "episodic" by definition, but their insurers may view them very differently, and the gap between 14 and 15 could be the gap between approval and denial for certain therapies.
Chronic migraine is not a permanent state, and neither is episodic. Transformation in both directions is documented and clinically meaningful.
Episodic to chronic transformation has well-established risk factors:
Roughly 2 to 3% of episodic migraine patients transform to chronic each year. At the high-frequency end, that rate is substantially higher.
Chronic to episodic reversal is also real, and this is where effective preventive treatment, including CGRP pathway therapies, demonstrates one of its most meaningful outcomes. A patient who begins preventive treatment at 18 headache days per month and drops to 9 after six months has not just had a good response; they have crossed a diagnostic threshold. They are now technically episodic. That reclassification matters for how their ongoing treatment is coded, justified, and renewed.
CGRP monoclonal antibodies, the class of preventive medications including erenumab, fremanezumab, galcanezumab, and eptinezumab, do not all carry identical FDA-approved indications. Some are approved specifically for chronic migraine, some for episodic, and some for both. The approval language matters because insurers anchor their prior authorization criteria to it.
| Category | Headache Days/Month | Typical Treatment Approach | Insurance Access Implications |
|---|---|---|---|
| Episodic low-frequency | Fewer than 8 | Acute treatment focus; preventive if quality of life warrants | Narrower access to injectables; some CGRP preventives may require step therapy |
| Episodic high-frequency | 8 to 14 | Preventive strongly considered; CGRP preventives may be indicated | Access depends on specific drug label; documentation of frequency is critical |
| Chronic | 15 or more, for 3+ months | Preventive treatment standard of care; multiple CGRP options indicated | Broader access to preventives; prior auth criteria generally align with this population |
When a prior authorization form asks for your diagnosis code and your monthly headache day count, those fields are doing real work. An insurer reviewing a request for a CGRP preventive that is approved only for chronic migraine will deny the request if your documented MMD (monthly migraine days) does not support that diagnosis, regardless of how disabled you are on the days you do have attacks.
If you are near a diagnostic threshold, at 13, 14, or 15 headache days per month, accurate documentation from a daily headache log could be the difference between qualifying for a preventive therapy and being denied. Memory-based estimates almost always undercount.
Most people cannot accurately recall whether they had 12 or 15 headache days across the past three months without a contemporaneous record. Studies on headache recall consistently show that patients underestimate frequency, particularly for shorter or lower-intensity attacks. A 90-minute headache that resolves with an OTC analgesic is easy to forget. But it counts.
Counting only your worst migraine attacks, the ones that kept you in bed, produces a number lower than your true headache day count. Counting every day you took any headache medication, including preventive-plus-rescue combinations, and every day you had any headache meeting the duration threshold, gives you the accurate clinical picture.
The goal is not to inflate the count to qualify for better treatment. It is to reflect reality. Undercounting means your documented disease burden does not match your actual disease burden, which leads to undertreated migraine and denied authorizations.
There is a specific scenario that catches patients off guard: a successful response to a CGRP preventive that drops headache days enough to shift the diagnosis from chronic to episodic. This is a good outcome clinically. But at renewal time, an insurer reviewing a patient whose current MMD is 8 may question why they need a medication indicated for chronic migraine.
The answer is in the baseline. A patient who began treatment at 18 headache days per month and is now at 8 because of that treatment has demonstrated exactly the kind of response the drug is supposed to produce. Without a log showing that baseline trajectory, not just current counts but the history of counts over time, making that case to an insurer is much harder. The 8 days per month looks like episodic migraine if you only have the last 90 days of data. The full longitudinal record tells the real story.
The episodic and chronic migraine classification system exists to match disease burden to treatment. But it only works if the underlying data is accurate. Daily logging, capturing every headache day, duration, severity, medication use, and any missed activities, is not a bureaucratic chore. It is the mechanism by which your actual disease state gets recorded in a form that a clinician can use, an insurer can evaluate, and you can point to when you need to make a case for yourself.
Whether you are trying to understand a new diagnosis, building documentation for a prior authorization, or tracking whether a preventive therapy is actually working, an accurate and current headache day count is the foundation everything else rests on.
Educational, not medical advice. Migraine Tracker: CGRP Log is a personal tracking tool, not a medical device. It does not diagnose, treat, or provide medical advice. Always talk to your clinician.
Chronic migraine is defined in international headache diagnostic criteria as 15 or more headache days per month for more than 3 months, with at least 8 of those days meeting criteria for migraine. The remaining days may be tension-type or other headache.
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