A Closer Look at H2 Receptor Antagonists 

• H2 receptor antagonists or histamine receptor antagonists are also known as antiulcer drugs. 
• These drugs are under the name Cimetidine, Famotidine, Nizatidine, and Ranitidine.
• Caution must excercized when administering these drugs to prevent adverse effects. 

What Are H2 Receptor Antagonists?

H2 receptor antagonists or histamine receptor antagonists are also known as antiulcer drugs.

Examples of these drugs are:

• Cimetidine
• Famotidine
• Nizatidine
• Ranitidine

When given food and antacids, the absorption of H2 receptor antagonists is impaired. They are distributed widely throughout the body, metabolized by the liver, and excreted primarily in the urine.

The acid secretion in the stomach depends on the binding of gastrin, acetylcholine, and histamine to receptors on the parietal cells. If the binding of any of these substances are blocked, it results in lower acid production.

As the name indicates, the H2 receptor antagonists bind to the histamine receptors in the stomach and reduce acid production. They are valuable in treating and preventing peptic ulcer disease because they decrease basal acid secretion, and food-stimulated acid secretion. 

 H2 receptor blockers are indicated in diseases, such as 

•  Short-term treatment of duodenal ulcer (DU) 
• Active benign gastric ulcer (GU) 
• Maintenance therapy of DU  
• Pathologic hypersecretory conditions e.g., Zollinger-Ellison syndrome 
• Gastroesophageal reflux disease (GERD) 
• Prevention of upper GI bleeding in critically ill patients

Examples of H2 Receptor Antagonists

The examples of H2 receptor antagonists, their available dosage forms, usual adult doses, and familiar brand names can be summarized as follows: 

Cimetidine 

• Available dosage forms: 200mg, 300mg, 400mg, and 800mg tablets and 300mg/5ml liquid 
• Usual adult dose: 200 to 400 mg BID 
• Common brand name: Cimetidine 

Famotidine 

• Available dosage forms: 10mg, 20mg, and 40mg tablets, 40mg/5ml oral suspension, and 10mg/ml injection 
• Usual adult dose: 20 to 40 mg BID 
• Common brand name: Pepcid 

Nizatidine  

• Available dosage forms: 15mg/ml oral suspension and 75mg, 150mg, and 300mg capsules  
• Usual adult dose: 150 mg BID 
• Common brand name: Axid 

Ranitidine  

• Available dosage forms: 75mg, 150 mg, and 300 mg tablets, 150 and 300 mg capsules, 15mg/ml syrup, and 25mg/ml injection 
• Usual adult dose: 150 mg BID 
• Common brand name: Zantac 

Side Effects of H2 Receptor Antagonists

Generally, H2 receptor blockers are tolerated well. Sometimes they may cause: 

•  Hepatitis 
• Hematologic toxicity 
• CNS effects such as headache, fatigue, and cognitive impairment 

 Cimetidine has common side effects of loss of sexual desire, impotence, and gynecomastia in men. It occurs due to the blockade of androgen receptors in breasts. Therefore you need to be cautious when giving cimetidine to male patients. 

Nursing Implementations

With H2 receptors antagonists, keep the following in mind:  

• If the dose is once daily, administer it at bedtime. However, for twice daily doses, you can give one in the morning and another in the evening. For multiple doses, you can administer them with meals and at bedtime.  

• Follow the recommended infusion guidelines of H2 receptor antagonists. Exceeding them can lead to adverse cardiovascular effects. A continuous infusion of these drugs is more effective in controlling acid secretion.  
• If antacids and H2 receptor antagonists are prescribed together, administer antacids at least one hour before H2 blockers. Giving them together can lead to drug interaction because antacids can impair the absorption of H2 receptor antagonists when given together.  
• In patients with renal disease, you need to adjust the dose.  
• Avoid stopping the drug abruptly, as doing that can lead to acid rebound because of increased gastrin levels. Before stopping the H2 receptor antagonists, start a proton pump inhibitor to prevent acid reflux.  

Drug Interactions of H2 Receptor Antagonists

Cimetidine might increase the blood levels of some drugs by reducing their metabolism in the liver and their subsequent excretion. 

 Those drugs are: 

•  Oral anticoagulants 
• Propranolol (and possibly other beta-adrenergic blockers) 
• Benzodiazepines  
• Tricyclic antidepressants 
• Theophylline 
• Procainamide 
• Quinidine 
• Lidocaine 
• Phenytoin 
• Calcium channel blockers 
• Cyclosporine 
• Carbamazepine 
• Opioid analgesics 

 Cimetidine, when given with carmustine, increases the risk of bone marrow toxicity.  It also inhibits ethyl alcohol metabolism in the stomach, resulting in higher blood alcohol levels. Therefore, ask the patient not to consume alcohol with cimetidine therapy. 

*Also note thae antacids reduce the absorption of cimetidine, nizatidine, ranitidine, and famotidine. Therefore administer them with a time gap of at least 1 hour.  

The Bottom Line

With H2 receptor antagonist therapy, monitor the patient for side effects such as headache and signs of GI bleeding, such as blood in the stool.  

Ask your patients not to take any H2 receptor antagonists for more than 8 weeks.  

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