Niclosamide is an effective therapeutic medicament for the treatment of endometriosis. Niclosamide treatment can reduce the size of endometriotic lesions and reduce cell proliferation in the lesions. Niclosamide treatment can result in the inhibition of proteins and genes associated with inflammatory signaling, including those associated with NFκB and STAT3 activation, but without altering the expression of steroid hormone receptors. Niclosamide can be effective in reducing the growth of endometriotic lesions in female mice without impairing their ability to become pregnant and without negative effect on gestation and offspring. The invention herein is useful in treating symptoms associated with endometriosis, such as reducing the growth of lesions of the endometrium, without disrupting normal reproductive function in females. The invention also relates to derivatives of niclosamide having improved solubility and enhanced efficacy.