🔴 DEFINITION

🔶Antihistamines:

Drugs that combat the histamine released during an allergic reaction by blocking the action of the histamine on the tissue.


🔶H₁ antagonists :

H₁ antagonists, also called H₁ blockers, are a class of medications that block the action of histamine at the H₁ receptor, helping to relieve allergic reactions.

🔶H2 antagonist :

H₂ antagonists, sometimes referred to as H2RAs and also called H₂ blockers, are a class of medications that block the action of histamine at the histamine H₂ receptors of the parietal cells in the stomach.

🔶Gynaecomastia :

Enlargement of male's breast due to harmonal imbalance.

🔶 Osteoporosis:

Low calcium level in the blood is called as osteoporosis caused due to thiazide diuretic also.

🔴 Classification Antihistaminics

🔺 H2 Antagonists

Cimetidine
Ranitidine
Famotidine
Roxatidine

🔺H3 Antagonists

Thioperamide (H1:H3 1:23000)
Impromidine (also H2 agonist
Tiprolisant
Clobenpropit

Note:- Though some selective H3 antagonists have produced, they have not found any clinical utility.

🔴MECHANISM OF ACTION🔴

📌ANTIHISTAMINES

◾Antihistamines are drugs which treat allergic rhinitis by and other allergies.
◾ Typically people take antihistamines as an inexpensive, generic, over-the-counter drug that can provide relief from nasal congestion, sneezing, or hives caused by pollen, dust mites, or animal allergy with few side effects.
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.
◾Consultation of a medical professional is recommended for those who intend to take antihistamines for longer-term use.

📌DIPHENHYDRAMINE

◾It is a highly sedative antihistaminic drug
◾Diphenhydramine mainly works through antagonizing the H1 (Histamine 1) receptor
◾Diphenhydramine acts as an inverse agonist at the H1 receptor, thereby reversing effects of histamine on capillaries, reducing allergic reaction symptoms. 

🔴ADVERSE DRUG REACTION🔴

📌 ANTIHISTAMINES

Common Adverse Effects
• Drowsiness
°Dizziness
• Impaired coordination
• Headache
• Epigastric discomfort
• Thickened bronchial secretions
• Dry mucous membranes
• CNS stimulation, paradoxical
• Constipation
• Euphoria
• Ataxia
• Dysuria
• Urinary retention
• Hypotension
• Blurred vision
• Diplopia
• Palpitations
• Tachycardia
• Photosensitivity
• Diaphoresis
• Erectile dysfunction
• Early menses
• Anorexia

Serious Reactions

Anaphylaxis/anaphylactoid reaction

• QT prolongation
• Anemia, hemolytic
• Thrombocytopenia
• Agranulocytosis
• Leukopenia
• Pancytopenia
• Arrhythmias
• Seizures
• Toxic psychosis
• Labyrinthitis, acute
• Heat stroke