Chronic daily headache is a type of frequently occurring headache which may be further classified as either:

Chronic migraine, previously known as transformed migraine, is a condition where episodic migraine-type headaches occur more frequently until the individual experiences a migraine on >15 days per month; or Medication overuse headache, which refers to a headache syndrome in which an individual who previously experienced episodic primary headaches does so with increasing frequency, and is unable to achieve appropriate pain relief with normal medication doses. Medication use thus increases and, paradoxically, contributes to headache pain.

Chronic daily headache is thought to most typically occur as a result of medication overuse. The increasing severity and frequency of headache symptoms is usually a withdrawal effect of addictive pain killers (e.g. aspirin, codeine) used to treat migraine or other types of primary headache (e.g. cluster headache). Frequent and regular use (and particularly overuse) of medications used to treat headaches can induce headaches and increase the individual’s requirement to use medication. However, some argue the order of events is reversed and that increasing frequency of headaches results in overuse of medication, rather than vice versa.

Chronic daily headache may also occur without medication overuse, when tension-type, cluster or migraine headaches become more frequent and intense. Some individuals with chronic daily headache who overuse medications fail to improve with withdrawal of the addictive medication, suggesting that medication overuse did not influence the onset of their chronic daily headaches.

Medication overuse and chronic migraine headaches, although once considered the same condition, are now diagnosed according to separate criteria. They are alternative subtypes of chronic daily headache. Chronic migraine is a relatively new term, which replaces transformed migraine and refers to headaches which are predominately migrainous in nature and occur in individuals who do not overuse pain relief medications. Those who overuse medication should not be diagnosed with chronic migraine but with medication overuse headache.