L25 Antihistamines and Allergic Emergencies →↑↓

Question 1

Effect, MoA and Side Effect of Antihistamines Diphenhydramine (First generation)

Answer 1

Relieve allergic rhinitis (seasonal allergy)
symptoms including sneezing, runny nose,
itching, and watery eyes

Relieve itching and swelling associated with uncomplicated allergic skin reactions.

Control coughs due to cold or allergy

Side Effects: fatigue, dizziness, and sedation.

MoA
an anticholinergic agent (specifically at the muscarinic receptor) and has the ability to penetrate the blood brain barrier due to their relative lipophilicity.

Not specific to the H1 receptor

low specificity, most agents possess weak anticholinergic and antiadrenergic side-effects
e.g. dry mouth, urinary retention, dizziness, blurred vision

Question 2

Name, Effect, MoA and Side Effect of Second generation antihistamines

Answer 2

Fexofenadine (non-sedative)

Specific for H1 receptors so no or little anticholinergic side effects

Relative rapid onset

longer duration of action (less frequent dosing)

SIde effects less common in second generation

Elimination Half-Lives:
→ Loratadine-up to 28 hours
→ Fexofenadine-14 hours
→Cetirizine-8 hours

Question 3

Therapeutic uses of antihistamines

Answer 3

Allergic and inflammatory conditions:
→controlling the symptoms of allergic rhinitis and urticaria
→controlling coughs due to cold or allergy
→ineffective in treating bronchial asthma

Motion sickness and nausea (prophylaxis):
→prevent or diminish vomiting and nausea
→the antiemetic action is due to their blockade of central
H1 and muscarinic receptors.
→→dimenhydrinate, hydroxyzine, promethazine theoclate

Somnifacients (inducing sleep):
→ many first-generation antihistamines, such as
diphenhydramine and doxylamine, have strong sedative properties and are used in the treatment of insomnia

Question 4

Drug Interactions of Antihistamines

Answer 4

→potentiation of the effects of all other CNS
depressants
→persons taking monoamine oxidase (MAO)
inhibitors should not take antihistamines
→decrease the effectiveness of cholinesterase
inhibitors in the treatment of Alzheimer’s disease

Overdoses
Effects in CNS
→hallucinations, excitement, ataxia, and convulsions.

Question 5

Treatment of anaphylaxis and MoA

Answer 5

Injection of Epipen (containing epinephrine) in thighs
→constrict blood vessel
→ dilate or relax smooth muscles in the lungs
→→ improve breathing, stimulates the heartbeat, and works to reduce the hives and swelling around the face and lips.

MoA
→inhibits the release of inflammatory mediators from mast cells
→stimulates the b2 -adrenergic receptors in bronchial smooth muscle causing bronchodilation →increases peripheral vascular resistance