Antihistamine
The vast majority of marketed H-antihistamines are receptor antagonists and only a minority of marketed compounds are inverse agonists at the receptor. Clinically, H-antihistamines are used to treat allergic reactions and mast cell-related disorders. Sedation is a common side effect of H-antihistamines that readily cross the blood–brain barrier; some of these drugs, such as diphenhydramine and doxylamine, are therefore used to treat insomnia. H-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 receptor.
Diphenhydramine
Diphenhydramine is an antihistamine mainly used to treat allergies. It is also used for insomnia, symptoms of the common cold, tremor in parkinsonism, and nausea. It is used by mouth, injection into a vein, and injection into a muscle. Maximal effect is typically around two hours after a dose, and effects can last for up to seven hours.
Common side effects include sleepiness, poor coordination, and an upset stomach. Its use is not recommended in babies. There is no clear risk of harm when used during pregnancy; however, use during breastfeeding is not recommended. It is a first generation H-antihistamine and works by blocking certain effects of histamine. Diphenhydramine is also an anticholinergic.
Diphenhydramine was first made by George Rieveschl and came into commercial use in 1946. It is available as a generic medication. The wholesale price in the developing world is about per dose. In the United States, it costs less than for a typical month’s supply. It is sold under the trade name Benadryl, among others.