Eight trials involving a total of 295 participants (265 in the double-blind trials) were included in the review.
All included trials scored at least 2 on the 5-point quality assessment scale. The 2 highest scoring trials scored 4 points. Both of these studies suggested that peppermint oil was not superior to placebo in the symptomatic treatment of IBS.
Five double-blind, placebo-controlled RCTs with a quality score of at least 3 were included in a meta-analysis. Two trials did not demonstrate a significant difference between the peppermint oil and placebo. Three studies showed that treatment with peppermint oil was significantly superior to placebo. Overall, this meta-analysis suggested a significant (p<0.001) positive effect of peppermint oil, compared with placebo, in the symptomatic treatment of IBS.
The placebo response ranged from 13 to 52% (mean 31%). The highest placebo responses (38 and 52%) were found in the 2 studies showing no difference between the treatment and placebo. In 3 studies, the placebo response varied significantly (p<0.01).
Five of the 8 trials reported the frequency of adverse drug reactions (heartburn, perianal burning, blurred vision, nausea and vomiting) while taking the peppermint oil. The frequency ranged from 11 to 36% (mean 20%) of the patients studied.