Benefits
Digestive comfort: abdominal pain and bowel symptoms (peppermint oil)
In a meta-analysis of 12 trials in people with IBS, peppermint oil more than doubled the chance of global symptom improvement versus placebo (about one extra person helped for every 3 treated) and eased abdominal pain. In the biggest individual trial (190 patients), the primary pain endpoint showed no significant difference, though pain scores improved. All tested peppermint oil, not menthol alone.
Sensation of easier nasal breathing (menthol lozenge)
In a double-blind trial, 62 people with a cold took an 11 mg menthol lozenge or a candy placebo. Menthol improved the feeling of nasal airflow within 10 minutes, but measured nasal resistance did not differ from placebo. This is a single trial, and an earlier study of inhaled menthol also found no consistent change in resistance. The effect is a sensation, not decongestion.
Mechanism of action
TRPM8 cold receptor activation
Menthol activates TRPM8, the cold-sensing ion channel, producing a cooling feeling without any change in temperature, as described in a pharmacology review. Stimulation of cold receptors in the nose explains why menthol increases the sensation of airflow without changing measured nasal resistance.
Gut smooth muscle relaxation (lab evidence)
In isolated tissue and animal studies, menthol relaxes intestinal smooth muscle, apparently by blocking calcium channels. This is the leading proposed explanation for peppermint oil's effects on cramping, but it was not measured in the human trials.
Other lab findings
Cell and animal studies report anti-inflammatory and mild antimicrobial actions, and weak effects on the 5-HT3 receptors involved in gut sensation. None of these was measured in people at supplement doses, so they remain hypotheses.
Clinical trials
Systematic review and meta-analysis of 12 randomized trials (Alammar et al. 2019, BMC Complement Altern Med)
835 patients with IBS; 7 trials (507 patients) analyzed for global symptoms and 6 (556 patients) for abdominal pain.
Peppermint oil improved global symptoms (RR 2.39, 95% CI 1.93 to 2.97) and abdominal pain (RR 1.78, 95% CI 1.43 to 2.20) versus placebo, with no heterogeneity. Number needed to treat was 3 for global symptoms and 4 for pain. Adverse effects did not differ significantly (9.3% vs 6.1%). The trials tested peppermint oil, not isolated menthol, and predate the largest single trial.
4-week randomized, double-blind, placebo-controlled trial of 180 mg three times daily, funded by IM HealthScience, the product's maker (Cash et al. 2016, Dig Dis Sci)
72 adults with mixed-type or diarrhea-predominant IBS (Rome III).
The Total IBS Symptom Score fell 40% on peppermint oil versus 24.3% on placebo at 4 weeks (p=0.0246), and the drop at 24 hours was also larger (p=0.0092). Several individual symptoms improved more than with placebo, with few adverse events. This is one small, maker-funded trial of a single formulation.
8-week multicentre randomized double-blind placebo-controlled trial of two 182 mg peppermint oil formulations (Weerts et al. 2020, Gastroenterology)
190 patients with IBS (Rome IV) at 4 hospitals in the Netherlands; 189 in the intent-to-treat analysis.
Neither formulation met the primary endpoint of abdominal pain response (46.8% and 41.3% vs 34.4% on placebo) or the co-primary overall relief endpoint. Small-intestinal-release oil did improve secondary measures of abdominal pain (p=0.016), discomfort (p=0.020) and IBS severity (p=0.020). Mild adverse events were more common with peppermint oil.
Double-blind randomized trial of an 11 mg menthol lozenge versus a candy placebo lozenge (Eccles et al. 1990, J Pharm Pharmacol)
62 people with nasal congestion from a naturally acquired common cold.
Measured nasal airway resistance rose in both groups over 2 hours with no difference between them. The menthol group reported a clear improvement in the sensation of nasal airflow 10 minutes after dosing, while the placebo group did not.