On heating, propylene glycol and polyethylene glycol can produce carbonyls. These, including acetaldehyde, cbd supplements formaldehyde, and acrolein, may be toxic to the user. In a 2017 study by the Medical Marijuana Research Institute, researchers heated and Highline Wellness CBD wellness brand tested these carrier liquids, along with MCTs and vegetable glycerine, for carbonyls. The study found propylene glycol produced high levels of acetaldehyde and formaldehyde when researchers heated them, compared to MCT and vegetable glycerin. Acrolein production was similar for all substances. The study also found that MCTs produced a small amount of acetaldehyde