H2 ANTAGONISTS

Question 1

what are H2 antagonists?

Answer 1

antihistamines - inhibit/block effects of histamine receptors

Question 2

what is histamine

Answer 2

NT
causes inflammatory and immune responses of the body
regulates physiological function in the gut

Question 2

examples

Answer 2

-idine
* rantidine (not recommended as slight traces of carcinogen NDMA found)
* cimetidine
* famotidine
* nizatidine

Question 3

MOA

Answer 3

H2 antagonists target H2 receptors

competitive reversible inhibit H2 receptors (GPCRs) on PARIETAL CELLS

reduce AC, reduce cAMP, reduce PKA activity,
less phosphorylation and activation of H+/K+ ATPase
reduces H+ secretion

Question 4

indications
(usually 2nd line after more effective PPIs)

Answer 4

GORD - reduce H+ production by blocking H2 histamine receptors in the parietal cells of gastric mucosa

peptic ulcer disease - GU,DU

chronic dyspepsia

H+ gastric acid reduction

Question 5

H2 blockers need to be taken

Answer 5

before dinner

Question 6

side effects

Answer 6

• GI upset - c,d
• dizziness
• fatigue
• headache
• myalgia
• skin reactions

less common - confusion, depression, erectile dysfunction, gynaecomastia, hallucinations

Question 7

interactions

Answer 7

cimetidine inhibits cytochrome P450 (CYP2C19)
so less drug clearance
eg. warfarin, clopidogrel (antiplatalet) increases - increased risk of bleeding

Question 8

contraindications

Answer 8

• pt at risk of gastric malignancy - H2RA mask symptoms of gastric cancer, so careful if red flag signs eg. weight loss, haematemesis
• fine in pregnancy/breast feeding - not harmful