Why do you get a headache when you fast? Tick your day and see the likely causes
A dull headache a few hours into a fast is one of the most common complaints from people new to fasting. Doctors recognise it: it usually starts after about 8 hours without food, is mild to moderate, and eases after eating.
Tick what fits your day, such as skipped coffee, little to drink, poor sleep or a long fast, and see the likely contributors and what tends to help, plus when a headache needs a doctor. It is not medical advice.
A dull headache a few hours into a fast is one of the most common complaints from people who are new to fasting. It is usually mild, it is rarely dangerous, and it often has more than one cause. Tick what applies to your day below to see which causes are likely and what tends to help. It is not a diagnosis, just a way to spot the pattern.
What is a fasting headache?
Doctors do recognise it. The International Classification of Headache Disorders describes a "headache attributed to fasting" as a diffuse, usually mild to moderate, non-throbbing headache that starts during a fast of at least 8 hours and clearly improves after eating. It is more common in people who already get migraines or other frequent headaches, and more likely the longer the fast goes on.
Why it happens is less settled. Researchers have suggested several contributors, and there is no single proven cause. That is why the checker below lists possibilities rather than one answer.
Tick your day, see the likely causes
Mild, dull and gone soon after a balanced meal is the typical fasting headache. Sudden, severe or strange is not, so get help.
What usually helps
Most fixes are small. Drink enough through the fasting window, keep your usual caffeine steady, sleep as well as you can, and do not jump straight to a long fast. When you do eat, a balanced meal with some protein, fibre and a little fat is a better start than a large pile of refined carbohydrate. And if a headache is bad, eat. A fast is a choice, not a test.
Over-the-counter painkillers are a question for your pharmacist or doctor, especially if you take other medicines. This guide does not give doses.
When it is not just the fast
NHS advice says to call 999 or go to A&E if a headache comes with sudden severe pain, weakness or numbness, confusion or drowsiness, trouble speaking or balancing, loss of vision, a seizure, a head injury or a rash that does not fade under a glass. Get an urgent GP appointment or call 111 for a headache with vision problems, vomiting, jaw pain when eating, a tender scalp, or one triggered by coughing, bending or exercise. Headaches that keep coming back, or that last beyond a few weeks even when you eat and drink well, also deserve a check.
Not medical advice. If you have a condition, are pregnant or breastfeeding, have or have had an eating disorder, or take medication (especially for diabetes or blood pressure), talk to a doctor before you try fasting. Intermittent fasting is not for everyone, and the evidence on its benefits is mixed.
Sources
- International Classification of Headache Disorders (ICHD-3), 10.5 Headache attributed to fasting
- Juliano & Griffiths, A critical review of caffeine withdrawal, Psychopharmacology, 2004
- Nordic Nutrition Recommendations 2023, fluid and water balance (EFSA adequate intake values)
- NHS, Headaches: causes and when to get help