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Peppermint oil solution was found to be effective for reducing gastric spasm during upper gastrointestinal endoscopy. The aim of the present study was to assess whether the gastric peristalsis-suppressing effect is dose-dependently induced by L-menthol, the major constituent of peppermint oil, and to determine the recommended dose of an L-menthol preparation.In this phase II, multicenter, double-blind, dose–response study, 131 eligible patients were randomly assigned to receive 20 mL of 0.4% L-menthol (n = 32), 0.8% L-menthol (n = 35), 1.6% L-menthol (n = 30), or placebo (n = 34). The primary efficacy measure was the proportion of subjects with no peristalsis in two time periods, 75 to 105 s after treatment and immediately before the completion of endoscopy.The peristalsis-suppressing effect of L-menthol increased dose dependently (5.6%, 32.0%, 47.4% and 52.9% in the 0%, 0.4%, 0.8% and 1.6% groups, respectively: P < 0.001, one-tailed Cochran-Armitage trend test). As compared with the placebo group, the proportion of subjects with no peristalsis after administration was significantly higher in the 0.8% group (P = 0.015) and 1.6% group (P = 0.009). Adverse events in the L-menthol dose groups occurred with similar frequencies in the placebo group.L-menthol suppresses peristalsis in a dose-dependent manner, and the dose–response reaches a plateau at 0.8% L-menthol. Further Phase III studies are needed to establish the superiority of 0.8% L-menthol over placebo.