Diacylglycerol O-acyltransferase 1 (DGAT1) plays an important role in synthesizing lipids, and inhibitors of DGAT1 have been investigated as potential treatments for diabetes and metabolic diseases. DGAT1 knockout (−/−) mice are resistant to obesity, have increased sensitivity to insulin, and exhibit sebaceous gland atrophy and alopecia. Prolonged pharmacological inhibition of DGAT1 with AZD7687 in mice results in the same skin phenotype, including sebaceous gland atrophy and alopecia, as seen in the skin of DGAT1 (−/−) mice. AZD7687-mediated effects on the skin were dose- and time-dependent and reversible. They occurred only at substantial levels of continuous DGAT1 inhibition. Prolonged treatment of dogs with AZD7687 also resulted in sebaceous gland atrophy but did not result in the more adverse skin changes of hair loss and skin lesions. Our findings highlight a significant risk of generating the same lesions that were seen in mouse skin during clinical development of DGAT1 inhibitors in humans and also reveal a species difference in the effects on the skin, indicating that the mouse may be an especially sensitive species. Therefore, although human therapeutic doses may not have the same influence on skin morphology as seen in mice, monitoring of skin changes will be essential in clinical trials with DGAT1 inhibitors.