The tea works by lowering the levels of circulating male hormones in these women, the scientists claim in the current online issue of Phytotherapy Research.
The authors called the findings preliminary.
"There are no recommendations for patients and physicians yet," said Mehmet Numan Tamer, co-author of the study and a professor of endocrinology and metabolism at Suleyman Demirel University in Isparta, Turkey. "This is the first clinical study about the spearmint tea, but further studies are needed to give the practical recommendations to patients."
But other experts feel the findings are far-fetched.
"This is absurd," said Khursheed Navder, an associate professor of nutrition and food science at Hunter College in New York City. "I completely think it is very preliminary, and you need major randomized trials. It's nothing to get thrilled about."
Hirsutism is a condition that can occur in both men and women, but, for obvious reasons, it is more of a cosmetic concern in women. The hair growth is a result of unnaturally high levels of androgens such as testoserone.
According to Tamer, standard therapy includes oral contraceptives to suppress androgens or drugs such as spironolactone, which prevents the body responding to androgens.
Spearmint, which grows naturally near Isparta, has been widely used for indigestion, nausea and vomiting, as well as for the common cold, cough, sinusitis, fever and bronchitis.
"Spearmint has been used as an herb for a long time now," confirmed Navder. "These are all folk remedies."
The Turkish researchers thought that spearmint might be linked with reports of diminished libido in townsmen (presumably because of its effects on androgen levels). In one previous rat study, spearmint reduced testosterone levels.
"Previously, we investigated the effects of [peppermint] and [spearmint] herbal teas on testicular function in an experimental rat model and found that testosterone levels were decreased," Tamer said. "Because we found the anti-androgenic effects of spearmint, we decided to observe the effect of this herbal tea on the androgen levels in hirsute women."
For this study, 21 women with hirsutism drank two cups of herbal spearmint tea for five days at a certain time in their menstrual cycle.
All women showed a decrease in free testosterone (circulating hormone not bound to other molecules) and an increase in several different "female" hormones, including follicle-stimulating hormone and estrogen.
There was no significant decrease in total testosterone levels; it was unclear if there was any change in amount of excess hair growth.
"The next step is to show the effect of spearmint tea on hirsutism scores of the women in a treatment period," Tamer said.
Although the amounts of spearmint used in this study are not generally toxic, the herb can be harmful if taken in larger amounts