Itching, sneezing, and inflammatory responses are suppressed by antihistamines that act on H1-receptors.In 2014, antihistamines such as desloratadine were found to be effective as adjuvants to standardized treatment of acne due to their anti-inflammatory properties and their ability to suppress sebum production.The dichotomy between 1st and 2nd generation antihistamines was introduced in 1983 at about the time of the European registration of two drugs: terfenadine and astemizole.

Histamine H1 receptor occupancy in human brains after single oral doses of histamine H1 antagonists measured by positron emission tomography.

Br J Pharmacology 1995; 116: 1649-1655 2 Rose C, Quache TT, Lorens C, Schwartz JC.

BMJ 2000; 320: 1184-1186 4 O'Hanlon JF, Ramaekers JG.

Antihistamine effects on actual driving performance in a standard test: a summary of Dutch experience, 1989-1994.

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receptor and heightens the receptor's activity; the receptor antagonists work by binding to the receptor and blocking the activation of the receptor by histamine; by comparison, the inverse agonists bind to the receptor and reduce its activity, an effect which is opposite to histamine's.-antihistamines can also reduce inflammation, since the expression of NF-κB, the transcription factor the regulates inflammatory processes, is promoted by both the receptor's constitutive activity and agonist (i.e., histamine) binding at the H A combination of these effects, and in some cases metabolic ones as well, lead to most first-generation antihistamines having analgesic-sparing (potentiating) effects on opioid analgesics and to some extent with non-opioid ones as well.

The most common antihistamines utilized for this purpose include hydroxyzine, promethazine (enzyme induction especially helps with codeine and similar prodrug opioids), phenyltoloxamine, orphenadrine, and tripelennamine; some may also have intrinsic analgesic properties of their own, orphenadrine being an example.

However there is no reason to believe that all 'non- sedating' antihistamines possess exactly the same low tendency to cross the blood brain barrier.

The study by Mann et al 3 nicely illustrates this point of view.

Histamine receptors exhibit constitutive activity, so antihistamines can function as either a neutral receptor antagonist or an inverse agonist at histamine receptors.