Pycnogenol, which is derived from the bark of maritime pine, which grows along the south west coast of France, already has a patent on its use to reduce pre-menstrual and menstrual pain.
It has previously been studied for its ability to reduce abdominal pain due to endometriosis.
The results of the new research build on this, and set up Pycnogenol as a potential side effect-free alternative to common hormone treatments used to address endometriosis.
The condition, which affects a million women in the US alone, is when the tissue that lines the uterus grows on the outside instead of the inside.
It is still shed each month, but is unable to leave the body, resulting in internal bleeding, inflammation and scar tissue.
In extreme cases, a sufferer may become infertile or even require a hysterectomy.
"Our results convey Pycnogenol as an extremely effective natural treatment without dangerous side effects," said lead researcher Takafumi Kohama.
The study involved 58 women aged 21 to 38 years, who had all undergone operations for endometriosis in the six months before the study commenced.
Regular menstruation and ovulation was confirmed during the three months prior to the study.
Kohama and his team at the Kanazawa University School of Medicine in Ishokawa, Japan, randomly divided the women into two groups.
One group received 30mg capsules of Pycnogenol twice a day, straight after morning and evening meals, for 48 weeks.
The other group received 3.75mg injections of the goadotropin-releasing hormone agent (Gn-RHa) leuprorelin six times every four weeks, for 24 weeks.
The women were examined at four, 12, 24 and 48 weeks to check for pain, urinary and bowel symptoms, and breakthrough bleeding.
At he beginnings, neither course showed any differences and women in both groups reported severe pain, pelvic tenderness and pelvic indurations.
After four weeks, Pycnogenols slowly reduced all symptoms from severe to moderate.
The Gn-RHa treatment reduced the symptoms more efficiently, but a relapse was reported around the 24-week mark.
What is more, Gn-Rha suppressed menstruation during treatment (an expected effect) and lowered oestrogen levels.
Of the women taking Pycnogenol, however, five actually fell pregnant during the study.
The earlier research on endometriosis-related abdominal pain, in which 80 per cent of the patients experienced a reduction of pain and 77 per cent disappearance of pain, was published in The European Bulletin of Drug Research .
The new study is due to be published in an upcoming edition of the Journal of Reproductive Medicine , according to Natural Health Science, the North American distributor of Pycnogenol.
The reference was not available at time of publication.