A study was conducted to compare the effect of ranitidine, cimetidine and placebo, given twice a day, on the 24-h intragastric acidity in 8 healthy individuals with a mean age of 25 years.

The double-blind cross-over study randomized the subjects to receive ranitidine 150 mg twice daily, cimetidine 400 mg twice daily and placebo at 08:00 h and 20:00 h.

• In comparison with placebo, gastric H⁺ concentration between 08:00 h and 19:00 h was reduced with cimetidine from 17.15 +61 (+ s.e. mean) mmol/l to 10.43 + 1.92 mmol/l (P < 0.01) with an inhibition of 39%.
• With ranitidine, it was redcued to 4.79 +10 mmol/l (P < 0.01 vs. placebo and cimetidine), with a 72% inhibition.
• Nocturnal H⁺ activity (20:00 h to 07:00 h) was reduced by cimetidine from 28.69 +44 mmol/I to 16.12 + 2.73 mmol/I (P < 0.01), with 44% inhibition.
• With ranitidine, it was reduced to 7.96 +86 mmol/l (P < 0.01 vs placebo and cimetidine) with an inhibition of 72%.

Ranitidine induced sustained inhibition lasting the entire duration of 24 hours, while cimetidine was less effective late in the afternoon and early in the morning.

Source: Dammann HG, Friedl W, Muller P, Simon B. Effect of ranitidine and cimetidine twice daily on the 24 h intragastric acidity in man. Br J Clin Pharmac. 1983;16(4):461-2.