Human Chemotherapeutic Agent, Fluorouracil (5-FU) is Deadly to Pets

Jun 25, 2019

The Neurology & Neurosurgery and Critical Care Departments at Red Bank Veterinary Hospital
would like to make all pet owners aware of a potential lethal toxicity harming our pets. Each year
we see dogs that have, unfortunately, ingested a human chemotherapeutic agent called 5-FU.
Fluorouracil, also known as 5-FU, is a human chemotherapeutic ointment used for skin
cancers such as superficial basal cell carcinoma, or actinic keratosis. Brand names of 5-FU
include Fluoroplex ®, Efudex ® and Carac ®. The ointment is applied to the affected skin,
and works by preventing synthesis of proteins in rapidly dividing cells, like tumor cells, which
cause death of the cancer.

Unfortunately, most prescribing physicians do not know to warn patients of the risk
associated with 5-FU to household pets. Additionally, there is absolutely NO warning on the
product packaging to keep the medication out of the reach of pets.

Who is Most at Risk?

We see ingestion of this medication most commonly in young dogs or puppies that
inadvertently chew on the tube containing the ointment. We have also examined dogs that die
from ingestion of 5-FU after simply licking the owner’s skin, which contains the ointment.


5-FU causes a buildup of ammonia in the body, which is harmful to the nervous system. Dogs
that have ingested 5-FU will present with an unsteady gait and whole body tremors that
rapidly progress to uncontrollable seizures and eventual death.


It is key to understand that even a VERY, VERY small amount of this material can be life
threatening to a puppy – usually less than 10% of the tube. A simple sharp puppy tooth is
all that is needed to pierce a tube of 5-FU and result in death of beloved household pet. The
key is to PREVENT the dogs from ingesting 5-FU in the first place.

If Your Pet Has Ingested 5-FU

In the event your dog ingests 5-FU, or you suspect ingestion, it is paramount that you do the following:

  • Immediately bring your pet to either your primary care veterinarian or a veterinary emergency hospital for an evaluation
  • Bring along any boxes or medication packaging, including the tube of medication so the veterinarian can use it to help assess the quantity of medication ingested
  • Safely keep the 5-FU out of reach of your pet during travel to the hospital

Pet Poison Prevention & Who to Contact

Curious pets have been known to get into things they shouldn’t. Be sure to pet proof your
home as much as possible to avoid inadvertent toxin exposure. In general, if you think your
pet has ingested any hazardous substances, including home and garden products, human
food and medications, poisonous plants, or other harmful objects, please contact your
veterinarian, your local veterinary emergency hospital, or the ASPCA’s 24-hour emergency
poison hotline at 1-888-426-4435. Keep these phone numbers in a convenient, easy to find
place at home. For a comprehensive list of hazards and toxins, visit

Finally, NONE of the manufacturers of 5-FU label the medication as toxic and deadly to
household pets. Furthermore, there is no label on the ointment tube or packaging with the
ASPCA 24-hour emergency poison hotline. We strongly encourage you to contact the
manufacturers of 5-FU to insist that appropriate labeling be included on the ointment tube
and packaging. Also, you may want to remind your physician of the potential harm of 5-FU
in household pets.

This information is courtesy of:

Eric N. Glass, MS, DVM, DACVIM

Neurology & Neurosurgery Department