17)Antihistamines Flashcards
Antihistamines
Most frequently used against sneezing + watery rhinorrhoea
- Can provide relief when person has nasal congestion
Classification - H1 antagonists
Gives relief of allergic reactions
- 1st gen ⇢ Pyrilamine, Antazoline, Clemastine
- 2nd gen ⇢ Cetirizine, Acrivastine
- 3rd gen ⇢ Levocetirizine , Carebastine, Deloratidine, Fexofenadine
Classification - H2 antagonists
Decrease secretion of stomach acid in cases of dyspepsia
- Cimetidine
- Ranitidine
- Famotidine
- Nizatidine
Mechanism of action of H1 receptors
histamine binds to various receptors (H1-H4) ⇢ Cinically we use H1 + H2 antagonists
CNS, smooth-muscle + endothelium
CNS
- Mediates sleep-wake cycle
- Mediates body temp
- Mediates nociception
- Mediates appetite
Periphery:
- Causes bronchoconstriction
- Smooth muscle contraction
- Vasodilation
- Hyper-nociception
Mechanism of action of H2 receptors
CNS, parietal cells, vascular smooth muscles, neutrophils, mast cells
Periphery:
- Vasodilation
- Stimulation of gastric secretion
Mechanism of action of H2 receptors
CNS, parietal cells, vascular smooth muscles, neutrophils, mast cells
Periphery:
- Vasodilation
- Stimulation of gastric secretion
H1 + H2 antagonists
H1 antagonists block periphery effects:
- They offer relief of allergic symptoms
- May also produce unwanted side effects
H2 antagonists ⇣ secretion of gastric acids
- Competitively bind to H2 receptors
- Inhibiting secretions of gastric mucosa
Pharmacokinetics
H1 receptor antagonists
- Well absorbed after oral administration
- 1st gen ⇢ distributed in all tissues
- Most metabolised in liver by P450 enzyme
- Cetirizine + Levocetirizine ⇢ excreted unchanged in urine
- Fexofenadine ⇢ excreted largely unchanged in faeces
Clinical use in dentistry
H1 antagonists
- Allergic rhinitis
- Allergic conjunctivitis
- Allergic dermatological conditions
- Nausea + vomiting
- Sedation - 1st gen only
H2 antagonists
- ⇣ Secretion of gastric acid
- Treatment of GI ulcer
Contraindications
During 1st trimester
- Makes birth defects more likely
1st gen contraindicated for drugs that result in CNS depression
- Barbiturates + alcohol
All antihistamines are safe during breastfeeding
Adverse effects
Dry mouth
Swelling
Drowsiness
Nausea + vomiting
Dizziness
