Antihistamines, h2-antagonists

Synonym(s)

H2 antagonists

Definition
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Drugs that block H2-receptors of the document cells in the stomach and lead to the healing of ventricular and duodenal ulcers by reducing acid secretion.

Indication
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H2-antagonists are used in dermatology as concomitant medication in systemic glucocorticoid, immunosuppressive or cytostatic therapy.

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Allergology

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Limited indication
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Pregnancy, nursing, renal failure.

Dosage and method of use
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S. Tab.

Undesirable effects
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Allergic reactions, CNS disorders, dizziness, blood formation disorders, eosinophilia, bronchospasms, gastrointestinal disorders.

Interactions
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S. Tab.

Contraindication
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Paragroup allergy (applies only to suspensions), sorbitol intolerance (applies only to suspensions), children and adolescents < 16 years, porphyria, severe liver dysfunction, anuria.

Tables
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Major interactions of H2 antagonists

Acenocoumarol	Bleeding tendency ↑
Barbiturates	Barbiturate levels ↑
Captopril	Neuropathy ↑
Carbamazepine	Carbamazepine levels ↑
Carmustine	Carmustine toxicity ↑, myelosuppression ↑
Chlordiazepoxide	Chlordiazepoxide levels ↑
Chlorpromazine	Fatigue ↑
Clomethiazole	Clomethiazole levels ↑
Digitoxin	Digitoxin toxicity ↑
Diltiazem	Diltiazem toxicity ↑
Iron	Iron level ↓
Erythromycin	Erythromycin toxicity ↑, hearing impairment ↑.
Halofrantine	Halofrantine bioavailability ↓, avoid combination.
Hydromorphone	Hydromorphone levels ↑
Isradipine	Isradipine levels ↑, RR drop, halve dose of isradipine
Itraconazole	Itraconazole levels ↓, take 2 hrs apart.
Ketoconazole	Ketoconazole level ↓
Labetalol	Labetalol level ↑
Levomethadone	Levomethadone level ↑
Lidocaine i.v.	Lidocaine level ↑, halve lidocaine dose.
Magnesium antacids	H2 antagonist levels ↓, take 2 hrs apart.
Mebendazole	Mebendazole levels ↑
Metformin	Metformin levels↑, lactic acidosis
Metoclopramide	H2 antagonist levels ↓, take 2 hrs apart.
Metoprolol	Metoprolol levels ↑, bradycardia
Nifedipine	Nifedipine level ↑, RR drop.
Nicotine abuse	H2 antagonist effect ↓
Opioid analgesics	Opioid toxicity ↑, seizures, confusional states, respiratory failure
Phenytoin	Phenytoin levels ↑.
Procainamide	Procainamide levels ↑, conduction disorders
Propanolol	Propanolol levels ↑, bradycardia
Rifabutin	Rifabutin levels ↑.
Sulfonylurea antidiabetics	Antidiabetic effect ↑
Terbinafine	Terbinafine levels ↑
Tetracyclines	Tetracycline effect ↓
Theophylline	Theophylline levels ↑
Valproic acid	Valproic acid level ↑
Warfarin	Bleeding tendency ↑
Zidovudine	Zidovudine toxicity ↑

Overview of the main H2 antagonists and their dosage.

Active ingredient	Dosage	Preparation
Cimetidine	400 mg/day p.o.	Tagamet
Famotidine	20 mg/day p.o. in the evening	Pepdul
Nizatadine	200 mg/day p.o. in the evening	Nizax
Ranitidine	300 mg/day p.o. in the evening	Sostril, Zantic