|8.1||Headache induced by acute substance use or exposure||G44.40|
|Coded elsewhere||10.3.6 Headache attributed to acute pressor response to an exogenous agent .|
This group of headache disorders can be caused 1) by an unwanted effect of a toxic substance, 2) by an unwanted effect of a substance in normal therapeutic use and 3) in experimental studies.
Substances that cause headache through their toxic effects, such as carbon monoxide, cannot be studied experimentally and the causal relationship between exposure and headache has therefore to be demonstrated in clinical cases where the substance has been used accidentally or for suicide attempt.
Headache as a side effect has been recorded with many drugs, often as just a reflection of the very high prevalence of headache. Only when it occurs more often after active drug than after placebo in double-blind controlled trials can headache be regarded as a true side effect. The double blind design can also be used experimentally to study the relationship between drug effects and headache. In some cases, for example NO donors, such studies have led to a deeper understanding of the involvement of neurotransmitter mechanisms in primary headaches. A number of substances such as NO donors and histamine induce an immediate headache in normal volunteers and in migraineurs. However, it is now clear that sufferers of primary headache also develop a delayed headache one to several hours after the inducing substance has been cleared from the blood.
Knowing the potential headache-inducing effects of substances in clinical use is important in order to label these substances appropriately. In general, migraine sufferers are much more susceptible to such headaches than other individuals and the same may be true for sufferers of chronic tension-type headache, episodic tension-type headache and cluster headache during cluster periods.
Paradoxically, the headache encountered by most people after heavy alcohol use may be a positive feature because it helps avoid excessive drinking.
Combinations such as alcohol and disulfiram may cause headache when individual agents might not.