An object of the present invention is to provide an agent for use in the effective treatment and prevention of endometriosis and uterine adenomyosis. The present invention relates to an agent for use in the treatment and prevention of endometriosis or uterine adenomyosis, comprising, as an active ingredient, a compound selected from the group consisting of 3-ethylpyridine, 3-methylpyridine, 2-ethylpyridine, and 2-methylpyridine.
