Details About Overdose or Poisoning Generic Salt ::  Cimetidine And Other H2 Blockers

Cimetidine and Other H2 Blockers

    

Drug Pharmacology ::

I. Pharmacology. Cimetidine, ranitidine, famotidine, and nizatidine are selective competitive inhibitors of histamine on H2receptors. These receptors modulate smooth muscle, vascular tone, andgastric secretions and may be involved in clinical effects associatedwith anaphylactic and anaphylactoid reactions as well as ingestion ofhistamine or histamine-like substances (eg, scombroid fish poisoning).Cimetidine, as an inhibitor of cytochrome P-450 enzymes, has beenproposed or studied in animals as an agent to block the production oftoxic intermediate metabolites (eg, acetaminophen, carbontetrachloride, halothane, Amanita mushroom poisoning), but thishas not been shown to be beneficial for human poisonings or toxicity.It is also an inhibitor of alcohol dehydrogenase (see Drug orLaboratory Interactions below) and has been suggested for use inpatients with an atypical aldehyde dehydrogenase enzyme to minimize adisulfiram (“Oriental flushing”) reaction from acute alcohol ingestion.

Drug Indications ::

Indications

   

Adjunctive with H1blockers such as diphenhydramine (see Diphenhydramine) in themanagement and prophylactic treatment of anaphylactic and anaphylactoidreactions (see Antivenom, Crotalinae [Rattlesnake], Antivenom,Latrodectus Mactans [Black Widow Spider], Antivenom, Micrurus Fulvius[Coral Snake], and Exotic Antivenoms).

Adjunctive with H1blockers such as diphenhydramine (see Diphenhydramine) in themanagement of scombroid fish poisoning (Food Poisoning: Fish andShellfish).

Ranitidinehas been used to reduce vomiting associated with theophyllinepoisoning. Because cimetidine may interfere with hepatic elimination oftheophylline, it should not be used.

Drug Contra-Indications ::

III. Contraindications. Known hypersensitivity to H2 blockers.

Drug Adverse Effects ::

IV. Adverse effects

   

Headache, drowsiness, fatigue, and dizziness have been reported but are usually mild.

Confusion,agitation, hallucinations, and even seizures have been reported withcimetidine use in the elderly, the severely ill, and patients withrenal failure. A case was reported of a dystonic reaction after IVcimetidine administration.

Areversible, dose-dependent rise in serum alanine aminotransferase (ALT)activity has been reported with nizatidine, a related agent. Hepatitishas also occurred with ranitidine.

Cardiacdysrhythmias (bradycardia, tachycardia) and hypotension have beenassociated with rapid IV bolus of cimetidine and ranitidine (rare).

E. Severe delayed hypersensitivity after high oral doses of cimetidine (case report).

F. Preparations containing the preservative benzyl alcohol have been associated with “gasping syndrome” in premature infants.

Drug Lab Interactions ::

Drug or laboratory interactions

   

Cimetidine,and to a lesser extent ranitidine, reduces hepatic clearance andprolongs the elimination half-life of several drugs as a result ofinhibition of cytochrome P-450 activity and reduction of hepatic bloodflow. Examples of drugs affected include phenytoin, theophylline,phenobarbital, cyclosporine, morphine, lidocaine, calcium channelblockers, tricyclic antidepressants, and warfarin.

Cimetidine,ranitidine, and nizatidine inhibit gastric mucosal alcoholdehydrogenase and therefore increase the systemic absorption of ethylalcohol.

Increasedgastric pH may inhibit the absorption of some pH-dependent drugs, suchas ketoconazole, ferrous salts, and tetracyclines.

Drug Dose Management ::

Dosage and method of administration. In general, there are no clinically proven advantages of any one of the H2blockers, although cimetidine is more likely to be associated withdrug-drug interactions. The lowest-strength dosage forms are availableover the counter, and several oral dosage form options (chewabletablets, oral solutions) may enhance palatability. See Table III–5 fororal and parenteral doses.

Drug Chemical Formulations ::

Formulations

   

Cimetidine (Tagamet, others)

   

1. Oral. 200-, 300-, 400-, and 800-mg tablets; 300-mg/5 mL oral solution (contains parabens and propylene glycol).

2. Parenteral. 150 mg/mL in 2- and 8-mL vials (Tagamet preparation has 0.5% phenol); premixed 300 mg in 50 mL saline (6 mg/mL).

Famotidine (Pepcid, Pepcid AC, Pepcid RPD)

   

1. Oral.10-, 20-, and 40-mg tablets; 10-mg chewable tablets and gelcaps; 20-and 40-mg disintegrating tablets; 40-mg/5 mL oral suspension (powder tobe reconstituted).

2. Parenteral.10 mg/mL in 1- and 2-mL single-dose and 4-, 20-, and 50-mL multidosevials (may contain mannitol or benzyl alcohol); premixed 20 mg in 50 mLsaline.

Ranitidine (Zantac, others)

   

1. Oral.75-, 150-, and 300-mg tablets and capsules; 15 mg/mL in 10 mL syrup(may contain alcohol and parabens); and 25- and 150-mg effervescenttablets.

2. Parenteral. 1.0 mg/mL in 50-mL container; 25 mg/mL in 2- and 6-mL vials (with phenol).

Nizatidine (Axid, others)

   

1. Oral. 75-mg tablets and 150- and 300-mg capsules.

2. Parenteral. Not available in this dosage form.

E. The suggested minimum stocking levels to treat a 70-kg adult for the first 24 hours (all are parenteral dose form) are the following:

   

1. Cimetidine. 1200 mg (8-mL vial).

2. Famotidine. 40 mg (4-mL multidose vial).

3. Ranitidine. 250 mg (10-mL vial).

Table III–5. Cimetidine, Famotidine, Nizatidine, and Ranitidine


Drug Route Dosea CimetidinePO300 mg every 6–8 h or 400 mg every 12 h (maximum 2400 mg/day). Children: 10 mg/kg single dose; 20–40 mg/kg/day.IV, IM300mg IV or IM every 6–8 h. For IV administration, dilute in normal salineto a total volume of 20 mL and give over 2 min or longer. Children: 10mg/kg.FamotidinePO20–40 mg once or twice daily (as much as 160 mg every 6 h has been used).IV20 mg IV every 12 h (dilute in normal saline to a total volume of 5–10 mL).RanitidinePO150 mg twice daily (up to 6 g/day has been used).IV, IM50 mg IV or IM every 6–8 h. For IV use, dilute in normal saline to a total volume of 20 mL and inject over 5 min or longer.NizatidinePO150 mg once to twice daily (or 300 mg once daily).

aNote: May need to reduce dose in patients with renal dysfunction.

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