Gonzalez and colleagues designed a placebo-controlled trial to compare the effects of single oral doses of cimetidine and ranitidine in maintaining intragastric pH above 2.5 and volume below 25 ml in ambulatory surgery patients who needed general anesthesia.

Patients were randomly allocated to receive either placebo, ranitidine 150 mg, or cimetidine 400 mg after waking up on the morning of surgery.

• At induction, both the treatment groups had significantly higher (p <0.05) gastric pH values compared to the placebo group. At extubation, the ranitidine group was found to have significantly higher (p <0.05) gastric pH compared to either the cimetidine or the placebo group.
• Both the treatment groups had lower gastric volumes in comparison with the placebo group at extubation (p <0.05).
• A larger number of patients were at risk for aspiration pneumonitis (pH <2.5 and/or volume >25 ml) in the cimetidine group compared to the ranitidine group (Figure).
• All patients in the placebo group were at risk for aspiration pneumonitis.

In conclusion, ranitidine may be the preferred agent over cimetidine as it has a more consistent effect on gastric pH and volume.

Figure. Patients at risk of aspiration pneumonitis in cimetidine and ranitidine groups

Source: Gonzalez ER, Butler SA, Jones MK, et al. Cimetidine versus ranitidine: single-dose, oral regimen for reducing gastric acidity and volume in ambulatory surgery patients. Drug Intell Clin Pharm. 1987 Feb; 21(2):192-5.