Jan 8 (Reuters) - An experimental drug being developed by AbbVie Inc and Neurocrine Biosciences Inc successfully reduced symptoms of endometriosis in premenopausal women in the first of two late-stage studies.

Endometriosis occurs when the lining of the uterus grows outside of the womb, leading to chronic pelvic pain. It is the leading cause of infertility.

After six months of treatment, both doses of the drug, elagolix, were found to significantly reduce menstrual and non-menstrual pain, compared with a placebo, AbbVie said.

(Reporting by Natalie Grover in Bengaluru; Editing by Ted Kerr)