Benefits
Overall gut comfort and bowel symptoms
In pooled randomized trials of adults with IBS, peppermint oil capsules improved overall symptoms more often than placebo (about 4 people treated for one to benefit in the most recent meta-analysis of 10 trials), but the evidence was rated very low quality. Two recent placebo-controlled trials did not meet their primary goals, although one improved secondary pain and severity scores.
Occasional abdominal discomfort and spasms
Pooled trials found peppermint oil eased abdominal pain more often than placebo (about 7 people treated for one to benefit in the most recent analysis). In one 72-person trial funded by the capsule's maker, symptom scores dropped more than with placebo within 24 hours. In animal gut tissue, the oil relaxed smooth muscle by reducing calcium entry into muscle cells.
Upper digestive comfort with a peppermint and caraway oil blend
In placebo-controlled trials lasting 4 weeks, adults with recurring upper abdominal pain and fullness who took an enteric-coated peppermint and caraway oil capsule twice daily reported less pain and fullness than with placebo. A Cochrane review rated the blend as probably giving a large improvement in overall symptoms (moderate certainty). Peppermint oil alone was not tested, and one trial was co-authored by the maker's staff.
Mechanism of action
Smooth-muscle relaxation through reduced calcium entry
In guinea pig and rabbit gut tissue, peppermint oil relaxed contracted intestinal muscle and blocked voltage-dependent calcium currents in smooth-muscle cells, an effect resembling dihydropyridine calcium-channel blockers. This antispasmodic action is the proposed basis for its effect on gut cramping; it was shown in animal tissue, not in people.
Clinical trials
Systematic review and meta-analysis of randomized placebo-controlled trials of peppermint oil in adults with IBS (Ingrosso et al. 2022, Aliment Pharmacol Ther).
10 randomized trials, 1,030 adults with IBS.
Peppermint oil beat placebo for global IBS symptoms (relative risk of not improving 0.65, about 4 treated for one to benefit) and abdominal pain (0.76, about 7 treated for one to benefit). Adverse events were more frequent with peppermint oil (relative risk 1.57), and the authors rated the quality of evidence as very low.
Double-blind, placebo-controlled trial at 4 Dutch hospitals in adults with IBS (Rome IV), comparing 182 mg small-intestinal-release peppermint oil, 182 mg ileocolonic-release peppermint oil and placebo for 8 weeks; secondary endpoints included IBS severity (Weerts et al. 2020, Gastroenterology).
189 adults in the intention-to-treat analysis (mean age 34).
Null on the primary endpoint: abdominal pain response did not differ from placebo (46.8% and 41.3% versus 34.4%), nor did overall relief. The earlier-release capsule did improve secondary measures of abdominal pain, discomfort and symptom severity. Mild adverse events were more common with both peppermint oil forms.
Randomized, double-blind, placebo-controlled trial at one US academic center, supported by the National Institutes of Health, of enteric-coated peppermint oil 180 mg three times daily for 6 weeks in adults with IBS (Rome IV) and moderate to severe symptoms; the primary outcome was the change in the IBS Severity Scoring System (Nee et al. 2021, Am J Gastroenterol).
Adults with moderate to severe symptoms, randomized 1:2 to peppermint oil or placebo.
Null: severity scores improved by 90.8 points with peppermint oil and 100.3 points with placebo (P = 0.97), and no secondary measure differed significantly between groups. Both groups improved substantially.
Randomized, double-blind, placebo-controlled multicenter trial of an enteric-coated capsule with 90 mg peppermint oil and 50 mg caraway oil (Enteroplant) for 4 weeks in non-ulcer dyspepsia; 45 enrolled and 39 evaluated for the main outcomes (May et al. 1996, Arzneimittelforschung).
45 adults with recurring upper digestive symptoms and no ulcer; 19 on the blend and 20 on placebo evaluated.
After 4 weeks, pain intensity and the doctor's global rating improved more with the blend than with placebo (p = 0.015 and 0.008). Adverse events: 4 with the blend and 3 with placebo. The trial was very small and tested both oils together, so it cannot show what peppermint oil does alone.
Randomized, placebo-controlled trial of one enteric-coated capsule (90 mg peppermint oil plus 50 mg caraway oil, Enteroplant) twice daily for 28 days in functional dyspepsia (May et al. 2000, Aliment Pharmacol Ther).
96 outpatients with recurring upper digestive symptoms (pain, pressure, heaviness or fullness); all evaluated.
Pain fell 40% from baseline with the blend versus 22% with placebo, and pressure, heaviness and fullness fell 43% versus 22%; 67% versus 21% were rated much or very much improved. All three main outcomes favored the blend. Six people reported adverse events (5 blend, 1 placebo). Peppermint oil was not tested alone.
Prospective, double-blind, multicenter randomized trial of Menthacarin (peppermint and caraway oil), 1 capsule twice daily, versus placebo for 4 weeks in functional dyspepsia; three co-authors worked for the maker, Dr. Willmar Schwabe (Rich et al. 2017, Neurogastroenterol Motil).
114 outpatients with chronic or recurrent upper digestive symptoms.
After 4 weeks, pain scores improved by 7.6 points with the blend versus 3.4 with placebo, and discomfort scores by 3.6 versus 1.3 points; the blend was ahead of placebo at 2 and 4 weeks (P < .001), and all secondary measures favored it. The capsule combined both oils, so peppermint oil's share is unknown.
Randomized, double-blind, multicenter equivalence trial comparing the enteric-coated blend (90 mg peppermint oil plus 50 mg caraway oil, twice daily) with the prescription drug cisapride 10 mg three times daily for 4 weeks in functional dyspepsia. With no placebo group it could only test whether the two looked similar (Madisch et al. 1999, Arzneimittelforschung).
120 outpatients enrolled and 118 evaluated (60 on the blend, 58 on cisapride).
Pain scores fell by 4.62 points with the blend and 4.60 with cisapride, which met the trial's test for equivalence. Adverse events were reported by 12 people on the blend and 14 on cisapride. Without a placebo arm, this does not show the blend did better than no treatment.