H₂ antagonists are the first class of highly effective drugs for the acid-peptic disease but have been surpassed by proton pump inhibitors (PPIs).
Four H₂ antagonists cimetidine, ranitidine, famotidine, and roxatidine are available everywhere and many others have been added subsequently.
They are primarily used in peptic ulcer, gastroesophageal reflux disease, and other gastric hypersecretory states.
Their interaction with H₂ receptors has been found to be competitive in case of cimetidine, ranitidine, and roxatidine, but competitive-noncompetitive in the case of famotidine.
Cimetidine was the first H₂ blocker to be introduced clinically and is described as the prototype, though other H₂ blockers are more commonly used now.
2 What to Know Before Using
If you are about to start a drug therapy, the risks, and benefits ratio of taking the medicine should be considered.
This is a decision that your doctor will make with your active participation. For this medicine, certain factors that may alter the drug action should be considered.
Present and past illness, drug interactions, hypersensitivity reactions, pregnancy, lactation and metabolic impairments should be considered cautiously.
On the contrary, the presence of some medical problems may affect the desired plasma concentration as well as the duration of action of the drug.
Make sure you tell your doctor if you have any other medical problems. Especially diabetes, heart disease, asthma, renal impairment and hepatic failure affect the use of any drug.
The H₂ blockers are used in conditions in which it is profitable to suppress gastric acid secretion.
Used in appropriate doses, all available agents have similar efficacy and equally good tolerability in the following medical conditions:
Duodenal ulcer: H₂ blockers produce rapid and marked pain relief within 2-3 days. 60-85% ulcers heal at 4 weeks and 70-95% ulcers at 8 weeks, but they are seldom used now to heal existing ulcers.
Gastric ulcer: healing rates obtained in gastric ulcer are somewhat lower (50-75% at 8 weeks). However, doses remain the same.
H₂ blockers can heal NSAIDs associated ulcers, but are less effective than PPIs or misoprostol.
Stress ulcers and gastritis: acutely stressful situations like hepatic coma, severe burns, and trauma, prolonged surgery, prolonged intensive care, renal failure, asphyxia neonatorum etc. are associated with gastric erosions and bleeding.
Mucosal ischaemia along with acid is causative. Intravenous infusion of H₂ blockers successfully prevents the gastric lesions and haemorrhage. It also promotes healing of erosions that have occurred.
Zollinger-Ellison syndrome: it is a gastric hypersecretory state due to a rare tumor secreting gastrin. H₂ blockers in high doses control hyperacidity and symptoms in many patients.
Gastro-esophageal reflux disease (GERD): H₂ blockers can afford symptomatic relief and facilitate healing of esophageal erosions, but are less effective than PPIs. They are indicated only in mild or stage-1 cases of GERD.
Prophylaxis of aspirin pneumonia: H₂ blockers are given preoperatively preferably evening or even before reducing the risk of aspiration of acidic gastric contents during anesthesia and surgery.
Other uses: they have adjuvant beneficial action in certain cases of urticaria who do not adequately respond to an H₁ antagonist alone.
Medicines should be used following the directions were given by a doctor. The dose of this medicine will vary according to patient’s condition or requirements.
You should follow the doctor's directions and advice. The amount of medicine that you take must not exceed the maximum therapeutic dose.
Also, the frequency of your daily drug administration and the duration of drug therapy depend on the particular medical problem for which you are taking the medicine.
The most commonly used H₂ antagonists are discussed below:
Ranitidine is a non-imidazole H₂ blocker which is about 5 times more potent than cimetidine.
Though its pharmacokinetic profile and plasma half-life is similar to cimetidine, a longer duration of action is obtained clinically because of high potency.
As it has no adrenergic action so it does not increase prolactin secretion. Due to lesser permeability into the brain, there is a lower propensity to cause CNS effects.
Dose schedule: For ulcer healing, 300 mg at bedtime or 150 mg twice in a day is given to the patients. The maintenance dose of ranitidine is regarded as 150 mg at bedtime.
0.1-0.25 mg/kg/hour by intravenous infusion has been used for prophylaxis of stress ulcers. In the case of parenteral use of ranitidine 50 mg is given intranuscularly or slow intravenous injection at every 6-8 hours.
Cimetidine: it is adequately absorbed orally, though bioavailability is 60-80% due to first-pass hepatic metabolism. Absorption is not interfered by the presence of food in the stomach.
It crosses the placenta and reaches milk, but penetration in the brain is poor because of its hydrophilic nature. About 2/3 of a dose is excreted unchanged in urine and bile, the rest as oxidized metabolites.
Dose schedule: For ulcer healing: 400 mg BD or 800 mg at bedtime orally; maintenance dose – 400 mg at bedtime.
Famotidine: a thiazole ring containing H₂ blocker which binds tightly to H₂ receptors and exhibits a longer duration of action. It is 5-8 times more potent than ranitidine.
The oral bioavailability of famotidine is 40-50%, and it is excreted by the kidney, 70% in the unchanged form.
Because of higher potency and longer duration, it has been considered more suitable for Z-E syndrome and for prevention of aspiration pneumonia.
Dose schedule: 40 mg at bedtime or 20 mg BD (for healing); 20 mg at bedtime for maintenance; up to 480 mg/day in Z-E syndrome; Parenteral dose – 20 mg i.v. 12 hourly or 2 mg/hr i.v. infusion.
Roxatidine: the pharmacodynamics, pharmacokinetic and side-effect profile of roxatidine is similar to that of ranitidine, but it is twice as potent and longer acting.
Dose schedule: 150 mg at bedtime or 75 mg BD; maintenance 75 mg at bedtime.
Always try to take your medicine in time. If you miss any dose of this medicine, you should take it as soon as possible.
But when it is time for your next dose, then skip the missed dose and go back to your regular dosing schedule.
You should store the medicine in a closed container at room temperature away from heat, moisture, and direct light. All kinds of medicines should be kept out of the reach of children.
Outdated medicine must be disposed of by an appropriate way.
4 Precautions to Take
The risks of taking a drug should be kept in mind during any drug therapy. Here, you should take part to make a decision for choosing a drug.
The risk and benefit ratio should be considered before starting a drug therapy. If you have had any allergic reactions to any medicine then you must inform your doctor about that.
Always be free to tell your physician if you have had any heart diseases or respiratory problems like bronchial asthma. Serious reactions may take place when it is used with another.
Here, cimetidine inhibits several cytochrome P-450 isoenzymes and reduces hepatic flow.
It inhibits the metabolism of many drugs so that they can accumulate to toxic levels e.g. theophylline, phenytoin, carbamazepine, phenobarbitone, sulfonylureas, metronidazole, warfarin, imipramine, lidocaine, nifedipine, quinidine etc.
Metabolism of propranolol and diazepam is also retarded, but this may not be clinically significant. Antacids reduce absorption of all H₂ blockers.
When used concurrently a gap of 2 hours should be allowed. Ketoconazole absorption is decreased by H₂ blockers due to reduced gastric acidity.
5 Potential Side Effects
There are some unwanted side-effects associated with each drug that usually do not need medical attention.
These side-effects usually go away during the treatment episode as your body adjusts to the medicine.
Additionally, your health care professional may advise you about the ways how to prevent or reduce those unwanted side-effects.
Sometimes you may need to consult with the doctor if you feel any serious discomfort.
The following side effects may warrant medical care immediately:
Cimetidine (but not other H₂ blockers) displaces dihydrotestosterone from its cytoplasmic receptor which causes an increase in plasma prolactin and inhibits degradation of estradiol by the liver. High doses given for long periods have produced gynaecomastia, loss of libido, impotence and temporary decrease in sperm count.
Transient elevation of plasma aminotransferases; but hepatotoxicity is rare.
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