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Pharmacokinetics

Absorption

Bioavailability

Rapidly absorbed after oral or IM administration.

Oral bioavailability: About 50%; similar in children 3.5–16 years of age.

Oral: Peak plasma concentration attained within 2–3 hours in adults and geriatric patients and within 1.6–2 hours in children 1 month to 16 years of age.

IM: about 90–100% absorption.

Commercially available oral solution, effervescent tablets, and conventional tablets are bioequivalent.

Duration

Following oral administration of a single 150-mg dose, substantial inhibition of gastric acid secretion reportedly continues for about 9.5 hours.

In pediatric patients, oral administration of 6–10 mg/kg daily (in 2 or 3 divided doses), maintained gastric pH throughout the dosing interval.

Following a single 150-mg oral dose, serum concentrations required to inhibit 50% of stimulated gastric acid secretion are maintained for up to 12 hours.

IM or IV: Following a 50-mg dose, serum concentrations required to inhibit 50% of stimulated gastric acid secretion are maintained for 6–8 hours.

Food

Food does not appear to substantially affect absorption or peak plasma concentrations.

Special Populations

Oral: In geriatric individuals, AUC may be substantially increased.

In individuals with cirrhosis, oral bioavailability appears to increase to about 70% and peak serum ranitidine concentrations appear to be higher because of reduced first-pass metabolism; considered minor, clinically unimportant.

Distribution

Extent

Widely distributed throughout body.

Distributed into CSF following oral administration; CSF concentrations in individuals with uninflamed meninges are about 3–5% of concurrent peak serum concentrations.

Distributed into human milk; milk concentrations appear to be 25–100% of concurrent serum concentrations.

Plasma Protein Binding

10–19%.

Special Populations

In individuals with cirrhosis, minor but clinically unimportant alterations in distribution occur following oral administration.

Elimination

Metabolism

Extensive first-pass metabolism after oral administration.

Metabolized in the liver to ranitidine N-oxide, desmethyl ranitidine, and ranitidine S-oxide.

Elimination Route

Excreted principally in urine.

Following oral administration, excretion of unchanged ranitidine in urine is dose-dependent; about 16–36% (unchanged) is excreted in urine within 24 hours.

Following oral administration, about 4% as ranitidine N-oxide, 1–2% as desmethyl ranitidine, and 1% as ranitidine S-oxide is excreted in urine within 24 hours.

Most of the urinary excretion occurs within the first 6 hours after administration.

The remainder of an orally administered dose is eliminated in feces.

Following IV administration, approximately 70% is excreted in urine as unchanged drug.

Half-life

Adults: Averages 1.7–3.2 hours and may be positively correlated with age.

Children 3.5–16 years of age: Averages 1.8–2 hours (range: 1.4–2.9 hours).

Neonates (<1 month of age): Averages 6.6 hours.

Special Populations

In patients with renal impairment, plasma clearance appears to be decreased and elimination half-life prolonged.

In patients with cirrhosis, minor but clinically unimportant alterations in half-life and reduced clearance occur following oral administration.

In geriatric individuals, clearance appears to be reduced and half-life prolonged because of decreased renal function; although half-life reported to be 3–4 hours following oral or parenteral administration in geriatric patients, in one clinical study it was about 6 hours following an oral 100-mg dose.