Colorless liquid used as a solvent and pH adjuster. Also used as a lather agent in skincare and haircare products when coupled with a foaming or detergent cleansing agent, such as cocamide DEA (which is not the same as pure DEA).
In 1999, the National Toxicology Program (NTP) completed a study that found an association between cancer and tumors in laboratory animals and the application of diethanolamine (DEA) and certain DEA-related ingredients to their skin. 
The National Toxicology Program (NTP) tested potential effects of DEA when a pure concentration of this ingredient was directly applied to mouse skin for a period of 14 weeks (minimum) and 2 years (maximum). The study reported no evidence of carcinogenicity when low doses (50–100 mg per kilogram of body weight) were used. Internal changes to organs (liver, kidneys) and external signs (inflammation, ulcers) were found as the dosages of DEA increased (up to 800 mg was used). 
For the DEA-related ingredients, the NTP study suggested that the carcinogenic response is linked to possible residual levels of DEA. However, the NTP study did not establish a link between DEA and the risk of cancer in humans and after evaluating the results of this study, the FDA ruled that there was no cause for concern with regards to DEA-related ingredients in cosmetics.  Although the results of this study are interesting, it is still unrelated to how DEA is used in cosmetics products and how consumers use them. In most instances, our contact with DEA in any form is brief, as it is present in rinse-off products, and it is not proven to cause harm to people. In vitro research on human skin samples has shown that DEA penetration is low, even under the condition of constant skin contact over a 24-hour period. 
In 2013, DEA was reevaluated and was considered safe for use in cosmetics at current levels and when ingredients known to form nitrosamines are not included in the formula. 
- National Toxicology Program. NTP Toxicology and Carcinogenesis Studies of Diethanolamine (CAS No. 111-42-2) in F344/N Rats and B6C3F1 Mice (Dermal Studies). Natl Toxicol Program Tech Rep Ser. 1999;478:1-212.
- U.S. Food and Drug Administration. Diethanolamine. [Internet]. 2006 [cited 2015 July]. Available from: http://www.fda.gov/cosmetics/productsingredients/ingredients/ucm109655.htm.
- Brain K, Walters K, Green D, Brain S, Loretz L, Sharma R, Dressler W. Percutaneous penetration of diethanolamine through human skin in vitro: application from cosmetic vehicles. Food Chem Toxicol. 2005;43(5):681-90.
- Fiume M, Heldreth B, Bergfeld W, Belsito D, Hill R, Klaassen C, Liebler D, Marks Jr J, Shank R, Slaga T, et al. Safety assessment of diethanolamides as used in cosmetics. Int J Toxicol. 2013;32(3S):26S-58S.