Ranitidine
**Ranitidine** is a competitive histamine H2-receptor antagonist and gastrointestinal prokinetic agent used widely in veterinary medicine. **Key Clinical Features:** * **Acid Suppression:** Used for the treatment and prophylaxis of gastric, abomasal, and duodenal ulcers, uremic gastritis, and stress-related or drug-induced erosive gastritis. * **Prokinetic Activity:** Uniquely among H2 blockers, ranitidine stimulates GI motility, making it useful for delayed gastric emptying and stimulating colonic activity in cats (e.g., for megacolon). * **Systemic Mastocytosis:** Helps manage hypersecretory conditions associated with mast cell tumors (which release massive amounts of histamine). > **Clinical Pearl:** Ranitidine is 3 to 13 times more potent than cimetidine on a molar basis. Furthermore, it has significantly fewer drug interactions because it does not appreciably inhibit hepatic cytochrome P450 enzymes, making it a safer choice for patients on multiple medications.
Mechanism: Ranitidine exerts its effects via two distinct mechanisms: 1. **Gastric Acid Suppression:** It competitively inhibits **histamine** at the **H2 receptors** located on the basolateral membrane of gastric parietal cells. * Histamine blockade โ decreased intracellular cAMP โ reduced activation of the **H+/K+ ATPase (proton pump)** โ significant reduction in both basal and stimulated gastric acid and pepsin secretion. 2. **Prokinetic Effect:** It reversibly inhibits **acetylcholinesterase (AChE)** in the gastrointestinal tract. * AChE inhibition โ decreased breakdown of acetylcholine โ increased **acetylcholine (ACh)** at muscarinic receptors โ stimulation of GI smooth muscle motility and increased lower esophageal sphincter pressure.
Dosing by species
- For ulcer disease/esophagitis ยท 2.5 mg/kg IV q12h or 3.5 mg/kg PO q12h ยท IV, PO ยท q12h
- For ulcer disease/esophagitis ยท 1-2 mg/kg ยท PO, IV, SC ยท q12h
- For ulcer disease/esophagitis ยท 2 mg/kg ยท PO, IV ยท q12h
- As a prokinetic agent to stimulate colonic motility ยท 1-2 mg/kg ยท PO ยท q8-12h
- As a prokinetic agent to stimulate colonic motility ยท 1-2 mg/kg ยท PO ยท q12h
- All uses ยท 2 mg/kg/day ยท IV ยท constant infusion ยท Not an effective acid suppressant in healthy cats.
- All uses ยท 2.5 mg/kg ยท IV ยท Not specified ยท Not an effective acid suppressant in healthy cats.
- For Helicobacter mustelae ยท 24 mg/kg ยท PO ยท q8-12h ยท 14 days ยท Uses Ranitidine bismuth citrate (must be compounded). Given with clarithromycin (12.5 mg/kg PO q8-12h).
- Foals ยท 1.5 mg/kg IV q8h or 6.6 mg/kg PO q8h ยท IV, PO ยท q8h ยท Often used with omeprazole (4 mg/kg PO q24h).
- Foals ยท 6.6 mg/kg IV q4h or 0.8-2.2 mg/kg IV four times a day; 5-10 mg/kg PO two to four times a day. ยท IV, PO ยท q4h or QID (IV); BID-QID (PO)
Routes of administration
Contraindications
- Hypersensitivity to ranitidine
- No specific contraindications available in the monograph
Adverse effects
- Vomiting (associated with rapid IV boluses in small animals)
- Pain at the injection site (IM administration)
- Mental confusion (rare, documented in humans)
- Headache (rare, documented in humans)
- Agranulocytosis (rare)
- Transient cardiac arrhythmias (if given too rapidly IV)
- Cardiac arrhythmias (rare, typically with rapid IV)
- Hypotension (rare, typically with rapid IV)
Drug interactions
- Acetaminophen ยท Ranitidine (dose-dependent) may inhibit acetaminophen metabolism.
- Antacids ยท High doses may decrease the absorption of ranitidine; administer at least 2 hours apart. ยท moderate
- Ketoconazole ยท Absorption of ketoconazole may be reduced secondary to increased gastric pH.
- Itraconazole ยท Absorption of itraconazole may be reduced secondary to increased gastric pH.
- Metoprolol ยท Ranitidine may increase metoprolol half-life and peak serum levels.
- Nifedipine ยท Ranitidine may increase nifedipine AUC by 30%.
- Propantheline ยท Delays the absorption but increases the peak serum level of ranitidine; relative bioavailability may be increased by 23%.
- Vitamin B-12 ยท Long-term ranitidine use may reduce oral absorption of Vitamin B-12.
- Sucralfate ยท May affect absorption; advisable to administer sucralfate 2 hours before H2 blockers ยท minor
- Digoxin ยท Ranitidine may reduce absorption or effect; stagger oral doses by 2 hours ยท moderate
- Metoclopramide ยท Ranitidine may reduce absorption or effect; stagger oral doses by 2 hours ยท moderate
Monitoring
- Clinical efficacy (resolution of clinical signs, improved appetite, absence of blood in feces/vomitus)
- Endoscopic examination (if indicated for ulcer healing)
- Serum ALT values (consider monitoring during high-dose, chronic IV therapy)
- Resolution of gastrointestinal clinical signs
- Heart rate and blood pressure (during IV administration)
Overdose
Clinical experience with ranitidine overdosage is limited, but the drug has a wide margin of safety. * **Signs of Toxicity:** In laboratory animals, massive doses (225 mg/kg/day) have caused muscular tremors, vomiting, and rapid respirations. Single doses of 1 gram/kg in rodents were not fatal. * **Treatment:** Handle using standard protocols for oral drug ingestions (e.g., gastric decontamination if recent and appropriate). Treat clinical signs symptomatically and supportively. Hemodialysis and peritoneal dialysis can effectively remove ranitidine from the body.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.