Antihistamine drugs Introduction Antihistamines are drugs that treat allergy symptoms by blocking the effects of histamines. Histamine is chemical released by your body during an allergic reaction and acts on a specific histamine receptor. There are four types of histamine receptor: H1, H2, H3 and H4 receptors. The term antihistamine only refers to H1 receptor antagonists. allergy symptoms: Congestion, runny nose, sneezing, or itching Swelling of the nasal passages Hives and other skin rashes Itchy, runny eye Mechanism Of Action Block action of histamine at H1 receptors. Compete with histamine for binding at unoccupied receptors. Cannot push histamine off the receptors if already bound. Types of Antihistamine 1: first-generation drugs sedating antihistamine drugs block peripheral H1 receptors, but also cross the blood – brain barrier and block central H1 and cholinergic receptors as well. Ex. diphenhydramine (Benadryl) - Dimetindene (fenistil) 2: second-generation drugs Non-sedating antihistamine drugs block peripheral H1 receptors, but do not cross the blood – brain barrier. Ex. Cetirizine (Zyrtec) - Loratadine (Alavert, Claritin) Diphenhydramine Diphenhydramine is in a class of medications called antihistamines 1st generation antihistamine Has a peripheral and central H1-antagonist action Sedating antihistamine Uses allergic symptoms - insomnia - common cold - Motion sickness Adverse side effects Side effects are due to CNS depression : Sedation - Dizziness - Euphoria - Blurred vision - Tinnitus Anticholinergic effects Contraindication hypersensitivity to diphenhydramine this drug should not be used in new-born or premature infants interaction Alcohol and other CNS depressants MAO inhibitors

2.  Antihistamines are drugs that treat allergy symptoms by
blocking the effects of histamines.
 Histamine is chemical released by your body during an allergic
reaction and acts on a specific histamine receptor.
 There are four types of histamine receptor: H1, H2, H3 and H4
receptors.
 The term antihistamine only refers to H1 receptor
antagonists.

4.  Block action of histamine at H1 receptors.
 Compete with histamine for binding at
unoccupied receptors.
 Cannot push histamine off the receptors if
already bound.

5. first-generation
drugs
sedating antihistamine drugs
cross the blood – brain barrier
block peripheral & central
H1 receptors
Non-sedating antihistamine
drugs
Not cross the blood – brain
barrier
block peripheral H1 receptors
Ex. Diphenhydramine (Benadryl)
Dimetindene (fenistil)

7.  1st generation antihistamine
 Has a peripheral and central H1-antagonist
action
 Sedating antihistamine
Diphenhydramine is in a class of medications called
antihistamines
Brand Names: Benadryl

8.  allergic symptoms
pruritusitching
 common cold  Motion sickness
 insomnia

9.  Side effects are due to CNS depression :
• Sedation
• Dizziness
• Blurred vision
• Tinnitus (ringing in the ear)
• Euphoria
• Anticholinergic effects

10.  hypersensitivity to diphenhydramine
 this drug should not be used in new-born or
premature infants
 Alcohol and other CNS depressants
 MAO inhibitors