This document discusses antihistamine drugs and how they work. It explains that antihistamines treat allergies by blocking the effects of the chemical histamine in the body. There are two major classes of antihistamines - H1-antihistamines which act on histamine H1 receptors, and H2-antihistamines which act on histamine H2 receptors in the stomach. H1-antihistamines are used to treat allergic reactions and conditions like insomnia, while H2-antihistamines are used to treat gastric acid conditions like ulcers. The document also provides classifications of different antihistamine drugs and their common uses.

1. ANTIHISTAMINIC
DRUGS
NIDHI SHARMA

2. Histamine-
– Histamine is an organic nitrogenous compound involved in local immune responses,
as well as regulating physiological function in the gut and acting as a
neurotransmitter for the brain, spinal cord, and uterus.
– Histamine is involved in the inflammatory response and has a central role as a
mediator of itching.
– Histamine - a chemical found in some of the body's cells - causes many of the
symptoms of allergies, such as a runny nose or sneezing. When a person is allergic to
a particular substance, such as a food or dust, the immune system mistakenly believes
that this usually harmless substance is actually harmful to the body.

3. .
• Once formed, histamine is either stored or rapidly inactivated by its primary degradative
enzymes, histamine-N-methyltransferase or diamine oxidase.
• In the central nervous system, histamine released into the synapses is primarily broken down
by histamine-N-methyltransferase, While in other tissues both enzymes may play a role. Several other
enzymes, including MAO-B and ALDH2, further process the immediate metabolites of histamine for
excretion or recycling.
• Histamine is derived from the decarboxylation of the amino acid histidine, a reaction catalyzed by
the enzyme L-histidine decarboxylase. It is a hydrophilic vasoactive amine.

4. • Bacteria also are capable of producing histamine using histidine decarboxylase enzymes unrelated to
those found in animals. A non-infectious form of foodborne disease, scombroid poisoning, is due to
histamine production by bacteria in spoiled food, particularly fish.
• Fermented foods and beverages naturally contain small quantities of histamine due to a similar
conversion performed by fermenting bacteria or yeasts. Sake contains histamine in the 20–40 mg/L
range; wines contain it in the 2–10 mg/L range.

7. Antihistaminic drugs-
– Antihistamines are drugs which treat allergic rhinitis and other allergies.
– Antihistamines can give relief when a person has nasal congestion, sneezing,
or hives because of pollen, dust mites, or animal allergy.
– Antihistamines are usually for short-term treatment. Chronic allergies increase the
risk of health problems which antihistamines might not treat,
including asthma, sinusitis, and lower respiratory tract infection.
The Two largest classes of antihistamines are-
1. H1-antihistamines
2. H2-antihistamines.
H1-Antihistamines- They act by binding to histamine H1 receptors in mast cells, smooth muscle,
and endothelium in the body as well as in the tuberomammillary nucleus in the brain.

8. They are used to treat-
• Allergic reactions in the nose (e.g., itching, runny nose, and sneezing).
• Insomnia.
• Motion sickness or vertigo caused by problems with the inner ear.
H2- Antihistamies- They bind to histamine H2 receptors in the upper gastrointestinal
tract, primarily in the stomach.
They are used to treat-
• Gastric acid conditions (e.g., peptic ulcers and acid reflux).

9. Group Generation Drug Uses Characteristics
H1 antihistamines First •Dimetindene
•Diphenhydramine
•Meclizine
•Doxylamine
•Promethazine
•Clemastine
•Dimenhydrinate
•Brompheniramine
•Hydroxyzine
•Antiemetic agent
•Sedative agent
•Anaphylactic shock
•Antiallergic agent
• Allergic
rhinitis and conju
nctivitis
• Contact
dermatitis
• Hay
fever, urticaria, an
gioedema,
and rhinorrhea
•Motion sickness
•Strong sedative action
•Anticholinergic side
effects
Second •Loratadine (desloratadine )
•Cetirizine (levocetirizine )
•Azelastine
•Fexofenadine
•Antiallergic agent
•Adjuvant
treatment in anaphylactic
shock
•Non-sedative/mildly sedative
H2 antihistamines •Ranitidine
•Cimetidine
•Famotidine
• Reduce production
of stomach acid
•Usually used as a second-
line treatment, or in
combination with PPIs
Classification of Antihistaminic Drugs-

10. Mechanism of Action-
– Blocks action of histamine at receptor
– Competes with histamine for binding
– Displaces histamine from receptor