RESULTS: Liquid Chromatography Tandem Mass spectrophotometry analysis of the TELSE tablet confirmed the presence of trimethoprim as the active compound. The TELSE tablet-treated females produced significant numbers of embryos with exencephaly (n = 8, 36.4%, *P trimethoprim once. The use of trimethoprim, a folic acid antagonist, peri-conceptionally increased the risk of exencephaly in the mouse.