The clinical features of drug-induced headache were first described in 1951 in relation to excessively frequent or daily use of ergotamine [
8]. Subsequently, the same authors described improvement after ergotamine withdrawal in 52 patients who had developed daily headache following daily intake of ergotamine [
9]. As from 1970s, drug-induced headache was also described in patients using analgesics and other compounds [
10]. There is now substantial evidence that all drugs used for the treatment of headache can cause MOH in patients with primary headache disorders [
10]. The pattern of use of the drugs that can lead to MOH varies substantially from country to country and is influenced by cultural factors. In many patients, it is difficult to identify a single causal substance, since 90% of patients take more than one compound at a time and each of the single drug may, in theory, induce headache.
Many terms have been used to describe MOH; in 1988 the condition was defined “drug-induced headache” by the International Headache Society [
11]. This term, however, was criticized since several drugs, other than painkillers, can cause headache, even after a single dose, (nitrates, for example). In addition, it has been questioned whether headache can be primarily drug-induced [
12]. Therefore, the headache associated with the use of symptomatic drugs was classified under the heading “Headache induced by chronic substance use or exposure”—to distinguish it from the group of headaches induced by the acute use of drugs—and included only two subtypes: headache induced by ergotamine and headache associated with analgesics. Studies conducted in the subsequent years, along with availability of new symptomatic drugs, suggested the need for a further revision of both the name and the diagnostic criteria [
13‐
15]. In 2004, the term “medication overuse headache” was finally introduced into the second edition of the International Headache Society’s Classification (International Classification of Headache Disorders—II edition, ICHD-II), also with the aim of emphasising excessive drug intake as the basis of this form of headache [
16]. In ICHD-II, MOH is classified amongst the secondary headache, within group 8: “Headache Attributed to a Substance or its Withdrawal”. Together, the 2004 classification [
16] and the two subsequent revisions of the diagnostic criteria for MOH [
17,
18] refine and extend the definition of this condition on the basis of both its chronicity (headache on more than 15 days/month for more than three months) and the different types of drugs overused, thereby identifying main types of MOH. In the case of ergotamine, triptans, opioids and combination medications in particular, intake on ≥10 days/month for >3 months is required, whereas simple analgesics are considered overused when they are taken on ≥15 days/month for >3 months (Table
1).