Antihistamine to reduce the effects of released histamines by their effectiveness.

1. Generations of Antihistamines

2. 1. First generation Antihistamines
• First generation antihistamines are the oldest H1-
antihistamines drugs. They are widely available and
inexpensive.
• They are effective in the relief of allergic symptoms.
• Adverse drug reactions are most associated with the first
generation H1-antihistamines. This is due to their relative
lack of selectivity for the H1 receptor and their ability to
cross the blood brain barrier.

3. First generation Antihistamines:
• First-generation oral antihistamines, including diphenhydramine
(Benadryl) and chlorpheniramine (Chlor-Trimeton), easily cross the
blood-brain barrier and affect H1 receptors in the central nervous
system (CNS).
• The first-generation antihistamine were the first type approved by
the Food and Drug Administration (FDA). They began to be
approved in the United States in the 1930s and are still prescribed
today.

4. First generation Antihistamines:
• Most notable about this generation of antihistamines is that they cross the blood-
brain barrier, which results in drowsiness.
• Examples of first-generation antihistamines:
• Brompheniramine • Chlorpheniramine
• Cyclizine • Doxepin
• Benadryl • Dramamine

5. Side effects of first-generation antihistamines:
Poor sleep
quality
Dizziness
Blurred
vision
Acute liver
damage
Dry
mouth
Nausea and
vomiting
Low blood
pressure
Constipation
and diarrhea

6. 2. Second generation Anti-histamines:
• Second-generation H1-antihistamines are newer drugs that are much
more selective for peripheral H1 receptors.
• This selectivity significantly reduces the occurrence of adverse drug
reactions, such as sedation, while still providing effective relief of
allergic conditions.
• Second-generation antihistamines were approved by the FDA and first
came to market in the 1980s. The second-generation antihistamines do
not cause drowsiness at standard dosage levels.

7. Continue….
• Second-generation antihistamines are considered to be safer than first generation
antihistamines because they do not cause drowsiness and interact with fewer
drugs.
• Examples of 2nd generation antihistamines:
• Cetirizine • Allegra
• Claritin • Clarinex
• fexofenadine • Loratadine
• Levocetirizine

8. Side effects of 2nd generation antihistamines:
Headache
Cough
Tiredness
Sore throat
Abdominal
pain
Vomiting
and nausea

9. 3. Third generation Antihistamines:
• Third generation H1 antihistamines are second generation antihistamines
formally labelled third generation because the active enantiomer
(Levocetirizine) or metabolite (Desloratadine) derivatives of second-
generation drugs intended to have increased efficacy with fewer adverse
drug reactions.
• Third generation antihistamine is generally used for treatment of allergic
rhinitis and chronic urticaria. Second and third generation antihistamines
do not have such significant effects on the CNS as first-generation types.

10. Continue…..
Examples :
• Norastemizole (Metabolite of astemizole)
•Decarboethoxy loratadine (Metabolite of loratadine)
• Levocetirizine (Active enantiomer of cetirizine)
• desloratadine (Clarinex)
• fexofenadine (Allegra)

11. 4. Treatment of some other diseases:
Insomnia Duodenal and gastric ulcers
Motion sickness Zollinger-Ellison syndrome
Anxiety Anorexia
Heart burn Headache
Parkinson’s disease Gastroesophageal reflux disease (GERD)
Anaphylaxis Vertigo

12. References:
 Randall KL, Hawkins CA. Antihistamines and allergy. Aust Prescr. 2018 Apr;41(2):41-45. Doi:
10.18773/austprescr.2018.013. Pub 2018 Apr 3.
 Simons FE, Simons KJ. H1 antihistamines: status and future directions. World Allergy Organ
J 2008; 1:145-55.
 Castillo M, Scott NW, Mustafa MZ, Mustafa MS, Azura-Blanco A. Topical antihistamines and
mast cell stabilisers for treating seasonal and perennial allergic conjunctivitis. Cochrane Database
Syst Rev 2015;1:CD009566.
 Carson S, Lee N, Thakurs S. Drug class review: newer antihistamines: Final report update two
[Internet]. Portland (OR): Oregon Health & Science University; 2010.