On heating, propylene glycol and polyethylene glycol can produce carbonyls. These, including acetaldehyde, formaldehyde, and acrolein, may be toxic to the user. In a 2017 study by the Medical Marijuana Research Institute, researchers heated and tested these carrier liquids, vitality vibes brand along with MCTs and vegetable glycerine, for carbonyls. The study found propylene glycol produced high levels of acetaldehyde and Vitality Vibes CBD support 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