Aromatherapy with Peppermint Oil for Morning Sickness: Placebo Effect Only?

Nausea with or without vomiting in pregnancy (NVP, also called morning sickness) affects 50-90% of pregnant women. Its most severe form, hyperemesis gravidarum, with a prevalence of 1.1%, causes great distress and disrupts life activities. Women who have hyperemesis gravidarum have a higher risk of preterm labor and preeclampsia. NVP may be caused by psychological and hormonal factors, changes in gastrointestinal (GI) motility, and/or Helicobacter pylori infection. Treatments, mainly aimed at reducing symptoms, ranging from dietary changes and oral pharmacological drugs to hospitalization with intravenous fluids and nutritional therapy. About half of the drugs used in NVP are in the US Food and Drug Administration (FDA)’s pregnancy drug Category C; that is, causing fetal damage in vivo and with no adequate or well-controlled human trials, but benefits may outweigh risks. To avoid such risks, pregnant women often use complementary and alternative medicine (CAM). Herbal medicine (HM), a biological CAM modality, includes aromatherapy (AT), the inhalation of volatile compounds from aromatic plant essential oils (EOs), among its branches. Midwives often use AT. Peppermint (Mentha piperita, Lamiaceae) is used in HM as an antispasmodic, carminative, antiemetic, lactation-enhancing, and sedative, and to treat respiratory and urinary tract infections, diabetes, dysmenorrhea, and NVP. Peppermint is classified in pregnancy drug Category B-2, has little history of use with pregnant or reproductive-age women and no well-controlled human studies, but also no significant harm reported in humans or animals. Peppermint EO (PEO), from the plant’s aerial parts, contains menthol, menthone, and methyl acetate. As a receptor channel 5-HT3 antagonist, it reduces nausea and vomiting. It is also used in AT to reduce fever and improve digestion. PEO is on the FDA’s list of substances generally regarded as safe (GRAS).

The authors conducted a single-blind, randomized, placebo-controlled clinical trial (RCT) of the effects of AT with PEO on the severity of NVP. Pregnant women referred to selected clinics of Shahid Beheshti University of Medical Sciences (Teheran, Iran) between December 2014 and the end of May 2015 (n = 95) were assessed for eligibility; 65 were enrolled. Inclusion criteria were an NVP score of 3-12 (mild to moderate) on the Pregnancy-Unique Quantification of Emesis and Nausea (PUQE) scale, based on frequency of nausea, vomiting, and retching and effects of NVP on quality of life; gestational age of six to 20 weeks; normal singleton pregnancy with no threat of miscarriage; and other factors including normal olfaction, no sensitivity to herbal agents, and no use of alcohol or tobacco (Nicotiana tabacum, Solanaceae). Use of other antiemetic agents was forbidden during the study. Demographic and obstetric questionnaires were completed at baseline. Women were randomized to placebo (n = 33) or PEO (n = 32). During the four-day intervention, five dropped out of the placebo group. In the PEO group, one was lost to follow-up, and three dropped out; of the latter, two were AT-intolerant. This left 28 in each group for analysis (total = 56).

PEO was distilled by Giah Esanse Phytopharm Co. (Golestan Province, Iran) diluted to 10% with sweet almond (SA; Prunus dulcis, Rosaceae) EO (Barij Esanse Pharmaceutical Co.; Kashan, Isfahan Province). SAEO was also the placebo. Identical dark bottles were used for both. Participants received a bottle of the appropriate agent, a dropper, cotton balls, four PUQE questionnaires, and instruction in AT self-administration. AT was to be used up to four times daily at the onset of nausea, with a PUQE questionnaire completed each evening.

P < 0.05 was considered statistically significant. There were no significant pre-intervention differences in the two groups in age, gestational age, education, job status, gravidity, or parity. All women in the study were housewives; most, in their second pregnancy. Gestational age was stratified for randomization. Mean NVP scores in the PEO group were 7.36 pre-intervention and 5.18 post-intervention; for the placebo group, 7.21 and 5.82, respectively. While this was a significant decrease in NVP scores in both groups (P < 0.001), the difference between mean scores was insignificant (P = 0.227). Adverse effects (AEs) were reported by four women in the PEO group and one in the placebo group (a statistically insignificant difference; P = 0.352); all were resolved in less than 24 hours and did not result in dropouts. While the safety of PEO AT in women with NVP is thus supported, and patients tended to be satisfied with the treatment (60.7% in the PEO group and 57.1% in the placebo group), PUQE scores showed no benefit for PEO over placebo. Other studies have used different AT methods and different endpoints. One using the Rhodes scale rather than PUQE scores, oil burner AT, and an active agent combining peppermint and lavender (Lavandula spp., Lamiaceae) EOs reported significant benefits after three days of therapy. In a study using inhalation AT for postoperative nausea, PEO, propyl alcohol, and saline all provided similar results. Researchers concluded that the anti-nausea effect was due to an AT impact on breathing patterns. Studies of controlled breathing with or without PEO report similar benefits for postoperative nausea. Studies of pregnant women are complex due to the volatile psychological status of many. AT, stimulating limbic areas of the brain and to an extent relying on individual aroma associations, may be particularly susceptible to such factors. Longer studies of larger size and investigating different forms of peppermint or other kinds of PEO AT might bring more clarity. Mental relaxation methods and breathing techniques seem to offer benefits in NVP and should be taught in prenatal classes.


Joulaeerad N, Ozgoli G, Hajimehdipoor H, Ghasemi E, Salehimoghaddam F. Effect of aromatherapy with peppermint oil on the severity of nausea and vomiting in pregnancy: a single-blind, randomized, placebo-controlled trial. J Reprod Infertil. January-March 2018;19(1):32-38.