Because histamines have been implicated in facilitating inflammatory processes (e.g., prostaglandin production), histamine antagonists would, therefore, be expected to reduce pain mediated by inflammatory processes (Raffa 2001). Diphenhydramine, hydroxyzine hydrochloride, hydroxyzine pamoate, and promethazine are among those that are commonly employed.
Used alone, antihistamines appear to have an analgesic ceiling effect. Histamine antagonists can augment opiate receptor binding of opioid analgesics (Rumore and Schlichting 1986) and therefore are often employed as co-administered adjuvant agents. The failure to observe substantial analgesia from the use of these agents alone, however, has largely restricted the use of histamine antagonists for persons with chronic pain who have other indications. These agents may be particularly useful in patients given their sedative, anti-emetic, antipruritic, and anxiolytic properties. They are generally well tolerated, with few respiratory or gastrointestinal side effects.
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