Antihistamines

Question 1

What is the mechanism of action for H1 antihistamines?

Answer 1

Competitive blockade of H1 receptors (antagonist)

Question 2

What are the clinical uses of H1 antihistamines?

Answer 2

• Allergic rhinitis
• Pollinosis
• Uticaria
• Anaphylaxis
• Off label uses: motion sickness, nausea, sleep aid

Question 3

What are the CNS effects that H1 antihistamines produce?

Answer 3

• Produce depression (sleepiness)
• Treat motion sickness

Question 4

What are the autonomic and peripheral nervous systems effects in H1 antihistamines?

Answer 4

Autonomic - Blocks constriction of respiratory smooth muscle

Peripheral - Blocks flare (wheals) and itch

Question 5

What do most antihistamines cause?

Answer 5

• Sleepiness/drowsiness
• Xerostomia (dry mouth)
• Dry mucus membranes

Question 6

What did older antihistamines cause?

Answer 6

Ventricular arrhythmias

Question 7

What are the 2 types of antihistamines?

Answer 7

1. Ethanolamines
2. Alkyamines

Question 8

What type of antihistamine is diphenhydramine (Benadryl)?

Answer 8

Ethanolamine

Question 9

What type of antihistmaine is chlorpheniramine (Chlor-Trimeton)?

Answer 9

Alkylamine

Question 10

Why is diphenhydramine (benadryl) included in the dental office emergency kit?

Answer 10

It can be used for allergic reactions/anaphylaxis - IV, IM or tablet

Question 11

Why would chlorpheniramine (Chlor-Trimeton) be included in the dental office emergency kit?

Answer 11

Allergic reactions/anaphylaxis - IM or IV

Question 12

What antihistamine is approved for treatment of all types of allergens and sold OTC?

Answer 12

Cetirizine (Zyrtec)

Question 13

What is the main side effect of cetirizine?

Answer 13

Causes sedation

Question 14

What are some of the non-sedating antihistamines?

Answer 14

• loratadine (Claritin) - OTC
• fexofenadine (Allegra) - OTC
• desloratadine (Clarinex) - (second generation version of Claritin)

Question 15

Whats the difference with sedating and non-sedating antihistamines?

Answer 15

Non-sedating antihistmaines have a longer onset time and last longer.

Question 16

What is the mechanism of action of H2 antihistmaines?

Answer 16

Competitive blockage of H2 receptors (GI tract)

Inhibits basal and nocturnal gastric acid secretion (evoked by histamine)

Question 17

What are H2 antihistamines clinically used for?

Answer 17

• Peptic ulcer
• Hypersecretion of gastric acid
• GERD
• Zollinger-Ellison syndrome (gastrin producing tumor causing hypersecretion of stomach acid)

Question 18

What are the H2 antihistamines? and what do they all end with?

Answer 18

Ends with “…tidine”

• cimetidine (Tagamet)
• famotidine (Pepcid)
• Nizatidine (Axid)
• Ranitidine (Zantac)

Question 19

What happens if you only take H2 antihistamines for peptic ulcers?

Answer 19

The antihistamines promote healing of peptic ulcers - but they will continue if the H pylori (which causes the ulcers) is not treated

Question 20

What’s the main H2 antihistamine adverse effect seen in elderly individuals?

Answer 20

Confusion, slurred speech, delirium and hallucinations

Question 21

What is the safest of all antihistamines used for GI indications?

Answer 21

ranitidine - Zantac

Question 22

What is the most dangerous antihistamine?

Answer 22

cimetidine - Tagamet

Inhibits cytochrome P450 to slow the clearance and increases the serum levels of many drugs - which increases the risk for overdose reactions

Question 23

What type of drug is cromolyn (Gastrocrom)?

Answer 23

Mast Cell Stabilizer

Question 24

How do mast cell stabilizers work?

Answer 24

They inhibit degranulation, preventing histamine release from IgE sensitized mast cells