Over-the-counter H2-receptor antagonists are frequently used; however, there is little information about their duration of action on human gastric acid secretion. Studies have reported inhibitory effects for up to 9 h after a dose.

A study by Grimley and colleagues assessed the profiles of 20-h intragastric acidity in 24 healthy subjects either given ranitidine 75 mg, cimetidine 200 mg or placebo. This was a three-way crossover study. Five millilitre aliquots of gastric juice were aspirated at half-hourly intervals during the day (0-10 h after the dose) and at hourly intervals overnight (10-20 h after the dose). Investigators measured the pH to three decimal places. Weighted intragastric acidity (AUC/time) was calculated for day- and night-time with 2.5-h intervals during the day and 5-h intervals at night. The results were expressed as mean weighted intragastric acidity (mmol/L).

The study findings were as follows:

• Daytime (0-10 h after dose)
• When placebo was given, the weighted intragastric acidity was 31.03. It decreased to 10.37 (P < 0.001 vs. placebo) and 16.23 (P < 0.001 vs. placebo) with ranitidine and cimetidine, respectively.
• Ranitidine was tied to inhibition of the weighted intragastric acidity to a greater extent compared to cimetidine (P < 0.001).
• Night-time (10-20 h after dose)
• When placebo was administered, the weighted intragastric acidity was 21.36. It came down to 16.65 (P < 0.001 vs. placebo) with ranitidine, but remaining unchanged at 20.03 (P = 0.886 vs. placebo) with cimetidine.
• Ranitidine could inhibit weighted intragastric acidity to a greater extent than cimetidine (P = 0.010).

A sub-analysis of the two 5-h intervals was done which revealed that in comparison with placebo, ranitidine led to a significant inhibition of the weighted intragastric acidity in the 10-15 h period.

To summarize –

In healthy individuals, the inhibitory effect of ranitidine on intragastric acidity can be seen up to 10-15 h following an oral dose. The inhibitory effect of cimetidine; however, is limited to the initial 10 h.

Source: Grimley CE, Constantinides S, Snell CC, et al. Inhibition of intragastric acidity in healthy subjects dosed with ranitidine 75 mg: a comparative study with cimetidine and placebo. Aliment Pharmacol Ther. 1997 Oct;11(5):875-9.