On heating, propylene glycol and polyethylene glycol can produce carbonyls. These, cbd hemp extract including acetaldehyde, formaldehyde, and acrolein, may be toxic to the user. In a 2017 study by the Medical Marijuana Research Institute, researchers heated and cbd capsules for anxiety tested these carrier liquids, along with MCTs and cbd capsules for anxiety vegetable glycerine, for carbonyls. The study found propylene glycol produced high levels of acetaldehyde and cbd supplements 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