We have recently received a number of enquiries relating to the safety of propylene glycol as an excipient in human medications that are intended for use in cats or dogs. Propylene glycol (E1520) is a humectant, preservative and solvent that is permitted in the UK as an additive in food stuffs as well as topical, oral and parenteral medication (both human and veterinary). It is often present as a solvent for the colour or flavour component of the product and therefore the quantity present in the final medicinal product will likely be very small. Propylene glycol can also be found in cosmetic products, electronic cigarette (vape) liquids and in some antifreeze products.
It is generally considered to be of low acute toxicity and poisoning (resembling ethanol intoxication initially and then lactic acidosis) is not likely unless large quantities (>5 ml/kg) are ingested; poisoning has occurred following ingestion of antifreeze products and high doses of IV preparations.
Chronic feeding studies have demonstrated haematological abnormalities (Heinz bodies with or without anaemia in cats and haemolysis in dogs), with cats being more sensitive than dogs, but the quantities involved in these studies are much larger than would be found in therapeutic doses of medications containing propylene glycol as an excipient.
Acute exposures to medications containing propylene glycol as an excipient would not present a risk to dogs or cats and it is likely that chronic low doses would also be well tolerated in dogs though caution would be advised in cats if long term use is intended.