Zantac
Zantac
Generic Name
Zantac
Mechanism
- Competitive antagonist of histamine H₂ receptors on parietal cells in the stomach lining.
- Blocks α‑amino‑β‑hydrogen‑linked (ATP‑dependent) signaling pathways, preventing the rise in intracellular cAMP that normally triggers gastric acid exocytosis.
- Leads to a reduction in basal, meal‑stimulated, and stimulus‑independent acid secretion by 60‑90 %.
Pharmacokinetics
- Absorption: Rapid oral absorption; C_max achieved 0.5–2 h after ingestion.
- Bioavailability: ~40–70 %, dependent on stomach pH.
- Distribution: Widely distributed; plasma protein binding 10–30 %.
- Metabolism: Extensive hepatic biotransformation (primarily via CYP450 6) → N‑dealkylated metabolites.
- Half‑life: 2.5–3 h (oral), 3–4 h (IV).
- Excretion: Renal (≈70 %) and biliary routes.
- Food effect: Reduced absorption in a high‑fat meal (≈30 % reduction).
Indications
- Gastric ulcers and duodenal ulcers – short‑term or long‑term therapy.
- Gastro‑oesophageal reflux disease (GERD) – symptom control and mucosal healing.
- Zollinger‑Ellison syndrome – adjunct to PPIs or when PPIs insufficient.
- Erosive oesophagitis (stage I‑II).
- Pre‑operative H₂ blockade in patients with known ulcer disease.
Contraindications
- Contraindicated in patients with hypersensitivity to ranitidine or any component of the formulation.
- Caution in patients with severe hepatic or renal impairment; dose adjustment may be required.
- Drug interactions:
- *Warfarin:* increases INR.
- *Ketoconazole:* ↑ ranitidine half‑life.
- *Cimetidine, other H₂ blockers:* additive hypomagnesemia risk.
- Adverse events: Rare but serious liver injury and acute kidney injury reported in post‑marketing surveillance.
Dosing
| Condition | Adult Dose | Children (≥6 y) | Notes |
| GERD, ulcers | 150 mg PO BID (or 300 mg qd) | 3 mg/kg PO BID (max 150 mg) | Dose may be increased to 450 mg qd based on response. |
| Zollinger‑Ellison | 150 mg PO BID | Often used with PPIs; adjust upward as needed. | |
| Post‑op prophylaxis | 150 mg PO qd | Used to reduce stress gastric ulceration. |
• Route: Oral tablet or liquid; IV infusion (0.3 mg/kg/h) in acute settings (e.g., severe bleeding).
• Administration with food: Preferred due to enhanced absorption.
Adverse Effects
Common (≥1 %)
• Headache, dizziness, fatigue.
• GI disturbances: nausea, diarrhea, abdominal discomfort.
Serious (≤1 %)
• Hepatotoxicity (transaminitis, jaundice).
• Acute interstitial nephritis; renal failure in rare cases.
• Severe hypersensitivity rash, anaphylaxis.
• Rare arrhythmias (QT prolongation) in patients with pre‑existing cardiac disease.
Monitoring
- Baseline and periodic liver function tests (AST/ALT, bilirubin).
- Renal function (serum creatinine, eGFR) every 4–6 weeks in chronic users.
- Magnesium levels if treatment >3 months to prevent hypomagnesemia.
- Coagulation profile (INR) if on concurrent anticoagulation.
Clinical Pearls
- Avoid prescribing Zantac as a “generic” without verifying quality; many batches previously contaminated with NDMA. Use validated products only.
- When used with PPIs (for Zollinger‑Ellison), monitor for rebound acid hypersecretion after PPI discontinuation.
- Ranitidine is a substrate for CYP3A4; patients on strong inducers (e.g., rifampin) may require higher doses.
- Rapid onset: Because of its competitive blockade, it can rapidly alleviate heartburn symptoms, making it useful for short‑term symptomatic relief.
- Patient education: Advise patients to take with meals to enhance absorption and to report any rash, jaundice, or severe abdominal pain immediately.
--
• *All information is up to date as of 2026 and reflects current pharmacologic understanding of ranitidine (Zantac).*