Antihistamines (Exam 2)

Question 1

What are the effects of histamine on the cardiovascular system?

Answer 1

Cardiovascular (H1 receptors):

1) Drop in blood pressure (vasodilation)
2) Tachycardia
3) Reddening of skin (vasodilation)
4) Throbbing headache (vasodilation of brain arterioles)

Question 2

What are the effects of histamine on the respiratory system?

Answer 2

Respiratory tract (H1 receptors):

1) Constricts respiratory smooth muscle
2) Difficulty breathing (think anaphylaxis)
3) Secretion of fluid and prostaglandin formation
4) Inflammation in airways

Question 3

What are the effects of histamine on the gastrointestinal tract?

Answer 3

Gastrointestinal Tract:

1) Secretion of HCl (gastric acid) = H2
2) Contraction of ileum = H1

Question 4

Compare and contrast the effects of antihistamines on H1 and H2 receptors.

Answer 4

1) Antihistamines are histamine receptor blockers (ANTAGONISTS)
2) H1 mechanism of action= competitive blockade of H1 receptors, ANTAGONISTS
3) H2 mechanism of action = competitive blockage of H2 receptors (GI tract)

Question 5

What are the most common side effects of antihistamines?

Answer 5

1) SEDATION is the primary adverse effect
2) Xerostomia
3) Dried up secretions in lungs make it difficult to “clear” residual secretions from upper respiratory tract
4) CNS effects can manifest differently in children & elderly

• May produce OPPOSITE effects: stimulation & excitation
• Symptoms include restlessness, excitability, & in severe cases, convulsions
• These symptoms may also occur in patients who take large doses of the drug (larger than recommended dose)

Question 6

What is the mechanism of action of H1 antihistamines?

Answer 6

Mechanism of action H1:

1) Competitive blockade of H1 receptors (H1 receptor blockers) = ANTAGONISTS
2) Drug occupies receptor site so that released histamine is unable to bind to receptor to produce an effect

Question 7

What is the mechanism of action of H2 antihistamines?

Answer 7

1) Competitive blockage of H2 receptors (GI tract)
2) Reduces intracellular concentration of cAMP; protein kinase (in parietal cells) that drives H+/K+-ATPase pump is shut down
3) INHIBITS basal & nocturnal gastric acid secretion (evoked by histamine)

Question 8

What are the Clinical uses H2 ?

Answer 8

GI disorders:
1) Peptic ulcer*

2) Hypersecretion of gastric acid*
3) GERD*
4) Zollinger-Ellison syndrome (gastrin producing tumor causing hyper secretion of stomach acid)

Question 9

What are the Clinical uses H1 ?

Answer 9

Other:
1) Allergic rhinitis

2) Pollinosis
3) Urticaria (localized edema with wheals)
4) Anaphylaxis
5) Off label uses: motion sickness, nausea/vomiting, dizziness, sleep aid, local anesthetics

Question 10

What are the use of antihistamines in dentistry?

Answer 10

1) Oral form (OTC tablets) for reversing minor allergic reactions (little 25 mg pink pills)
2) Injected drug in office emergency kit for minor/major allergic reactions/anaphylaxis
3) In liquid form as a topical anesthetic
4) liquid Benadryl is good for mucositis pain
5) May be (but rarely) injected as a local anesthetic
6) Similar structure & function to local anesthetic agents; used for patients who are allergic to local anesthetics
7) Oral form for pre-operative sedation

Question 11

What are the popular non-sedating antihistamines?

Answer 11

1) loratadine (Claritin) – OTC * know these
2) fexofenadine (Allegra)*
3) desloratadine (Clarinex)*
4) Slower rates of onset (1-3 hours)
5) Last longer duration (12 to 24 hours)
6) Long half-life (taken once or twice daily, depending upon agent)
7) Major side effect = headache

Question 12

What are the popular sedating antihistamines?

Answer 12

1) Ethanolamines= diphenhydramine (Benadryl
2) Alkylamines= chlorpheniramine (Chlor-Trimeton)
3) cetirizine (Zyrtec)*

Question 13

Describe the popular sedating antihistamine, chlorpheniramine (Chlor-Trimeton)?

Answer 13

Alkylamines= chlorpheniramine (Chlor-Trimeton)

1. In dental office emergency kit for allergic reactions and/or anaphylaxis
2. 10 mg/ml solution for injection
3. IM or IV = 4 mg tid or qid for up to 40 mg/day
4. Also available OTC for use in management of seasonal allergies

Question 14

Describe the popular sedating antihistamine, cetirizine (Zyrtec)*

Answer 14

cetirizine (Zyrtec)*

1. Popular OTC antihistamine
2. Approved for treatment of all types of allergens (animals, pollens, smoke, dust, etc.)
3. Used for perennial and seasonal allergies
4. Causes sedation

Question 15

Describe the popular sedating antihistamine, diphenhydramine (Benadryl)

Answer 15

Ethanolamines= diphenhydramine (Benadryl):

1. Injected drug in dental office emergency kit (allergic reactions/anaphylaxis)
2. IV or IM = 10 mg/ml or 50 mg/ml solution
3. Administer 10-50 mg to a maximum daily dose of 400 mg
4. Primary ingredient in some non-habit forming sleep aids Sominex and Nytol
5. Used “off-label” (unapproved use by the FDA) for: insomnia (as a sedative, sleep aid), motion sickness (similar to Dramamine), nausea and vomiting

Question 16

What are the popular H2 receptor blockers for gastrointestinal use?

Answer 16

1) H2 receptor blockers

a) cimetadine (Tagamet)* MOST adverse reactions, INHIBITS cytochrome P450 to INCREASE serum levels of many drugs, INCREASES risk for overdose reactions
b) famotidine (Pepcid)*
c) nizatidine (Axid)*
d) ranitidine (Zantac)* is the SAFEST & MOST recommended drug in this class (fewest drug interactions)

2) ALL of these drugs are equal in their efficacy to promote healing of peptic ulcers
3) Ulcers will recur upon discontinuation of drug if caused by H pylori (need antibiotic)
4) Sold OTC to relieve heartburn