Spearmint is a common garden plant capable of becoming invasive if not contained. You can brew the fresh leaves into tea or use them to complement the flavor of other beverages, such as a mint julep. Spearmint oil is a common flavoring found in toothpaste, mouthwash, ice cream and candy. While spearmint is generally safe for consumption and topical use, there are some potential side effects associated with this herb.

Herb Profile

Spearmint, scientifically referred to as Mentha spicata, is also known by various common names, such as sage of Bethlehem, Our Lady’s mint, lamb mint, curled mint and spire mint. The herb is indigenous to Europe and southwest Asia, although it is widely cultivated throughout the world. As the name implies, spearmint features pointed leaves that resemble spears.

Chemical Composition

Spearmint foliage contains up to 80 percent carvone, an organic compound that belongs to a class of chemicals called terpenoids, which are common to the essential oils of certain plants. This is the component responsible for the characteristic aroma and flavor of spearmint leaves. Other agents found in the herb include rosmaric acid, limonene, 1-8-cineole and thymonin.

Allergic Reactions

According to the “Physicians’ Desk Reference for Herbal Medicines,” the presence of carvone may induce sensitivity in certain people who use the oil topically -- or who simply brush their teeth with mint-flavored toothpaste. This condition, a form of contact urticaria called angular cheilitis, specifically involves inflammation of the lips and is caused by an allergy to plants in the mint family, including spearmint. Sometimes, mint allergy is difficult to detect. To illustrate, a case study published in the February 2001 issue of the "Australasian Journal of Dermatology” featured a 26-year old woman who experienced dermatitis of the lip for an entire year, yet she tested negative for allergy to her toothpaste. Finally, the link to mint was discovered after she reacted to a mint-flavored tooth powder and, subsequently, direct contact with mint leaves.

Sedative Effects

Some of the compounds found in spearmint are beneficial. For instance, an article published in the June 6, 2011, issue of "Toxicology and Industrial Health" reported that spearmint is a source of natural antioxidants. Some of these compounds, namely carvone, also produce an anti-nociceptive effect, meaning they reduce response to painful stimuli. The authors of a study published in the May 31, 2008, issue of the “Biological and Pharmaceutical Bulletin” observed this effect in mice and concluded that carvone reduces peripheral nerve excitability. The “Physicians’ Desk Reference for Herbal Medicines” states that carvone has demonstrated a neurodepressive effect in animals evidenced by an increase in the duration of sleep periods. This may explain why mint is used in aromatherapy and massage to relive headache and reduce stress. However, this also suggests that spearmint may potentially increase the effects of medications designed to treat insomnia and depression.