What is 5-FU (5-fluorouracil)?

5-FU is a chemotherapy drug that works by inhibiting cell growth. It is highly toxic to both dogs and cats — if your pet has ingested or been exposed to it, seek veterinary attention or advice immediately.

How soon after exposure will symptoms appear?

The timeline varies — vomiting or diarrhea may appear within a few hours, while neurological symptoms such as seizures can develop within hours to a day after exposure. If you have any concerns, bring your pet to a vet as soon as possible.

What should I prepare before taking my pet to the vet?

Note down the time of exposure, the number of vomiting and diarrhea episodes, food and water intake, and any current medications. Bring any remaining medication or its packaging so the vet can confirm what your pet was exposed to.

What is the prognosis for 5-FU poisoning?

The prognosis depends on the dose ingested and how quickly treatment is started. Early detection and proactive monitoring generally lead to better outcomes. Respiratory or neurological signs require urgent veterinary attention.

How much do the tests and treatment usually cost?

Costs vary by clinic and region — ask your vet for an estimate before proceeding.

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Disease Guide

5-FU (5-Fluorouracil) Poisoning in Dogs & Cats: Symptoms, Diagnosis & Care [2025]

5-Fluorouracil is a chemotherapy drug. If a pet accidentally ingests or is exposed to it, serious symptoms such as vomiting, diarrhea, and seizures may occur — this is a medical emergency requiring immediate veterinary attention.

Take 1 minute to check the symptoms first

Last updated: 2026/01/04

Pethealthmap Editorial Team

Sources: ASPCA, Merck

When to See a Vet

Seek Emergency Care Immediately

•Rapid or labored breathing, or blue/purple tint to lips or gums
•Continuous or repeated seizures accompanied by loss of consciousness
•Sudden collapse with inability to stand or unresponsiveness

Schedule a Clinic Visit Soon (within 24–72 hours)

•Noticeably depressed or significantly reduced activity level
•Repeated vomiting or high frequency of vomiting in a short period
•Acute onset of diarrhea or blood in stool

01What symptoms indicate something is wrong?

5-FU (5-fluorouracil) toxicity in dogs and cats most commonly presents with gastrointestinal and neurological signs. The condition can deteriorate rapidly — seek veterinary assessment promptly if you notice any of the following.

Lethargy, noticeably reduced activity, or excessive sleepiness
Repeated or uncontrollable vomiting
Acute diarrhea or blood in the stool
Rapid breathing or signs of respiratory distress
Seizures, tremors, or involuntary muscle twitching

Seek emergency care immediately if you notice the following symptoms:

⚠️ Difficulty breathing or blue/purple discolouration of the lips or gums
⚠️ Repeated or continuous seizures with loss of consciousness
⚠️ Sudden collapse and inability to stand up, or complete unresponsiveness

02How can you assess the situation at home? Key observations and when to seek emergency care

Observing and recording key changes at home before arriving at the clinic helps the vet quickly determine the timeline and severity of the poisoning.

Note the exact time of suspected exposure or onset of symptoms (to the nearest minute)
Record the number of vomiting episodes and the colour and content of the vomit
Track the frequency of diarrhea, stool consistency, and whether blood is present
Monitor breathing rate and watch for any wheezing or difficulty breathing
Record changes in appetite, water intake, and activity level
If seizures occur, film them on your phone as a reference for the vet

03How will the vet diagnose this? Common tests and examinations

The vet will combine a physical examination with blood and imaging tests to assess the extent of the toxicity, and may refer to a specialist or arrange close monitoring if needed.

Full physical examination

assessing vital signs and neurological status

Complete blood count (CBC)

evaluating changes in blood cell counts and signs of infection

Blood glucose and blood gas analysis

ruling out metabolic or respiratory dysfunction

Blood biochemistry panel

monitoring liver and kidney function and electrolyte imbalances

Imaging or specialist referral may be arranged if further evaluation is required

04What are the treatment and care steps after diagnosis? Medications, diet, and home care guide

Management may involve acute supportive treatment followed by ongoing monitoring. Owner compliance with follow-up appointments and careful record-keeping helps the vet track recovery progress.

Hospitalisation for observation and supportive care may be required during the acute phase
Blood and biochemistry parameters will be monitored to assess organ damage
Follow-up blood tests and clinical reassessment as directed by the vet
Short-term dietary adjustments with records of food intake and bowel movements
Close behavioural monitoring is needed if seizures or neurological signs are present
Ongoing care from a hospital or specialist experienced in toxicity cases is recommended

05What should you prepare before the vet visit? Key information to help the vet assess quickly

Bring a written record of the suspected exposure time and any subsequent symptom changes

List the number of vomiting and diarrhea episodes with specific observations

Prepare notes on recent water and food intake

Bring a list of current medications, supplements, and anything the pet may have been exposed to

If any remaining medication or packaging is available, bring it along for the vet to confirm

Use a secure carrier and arrange a consultation or emergency visit as soon as possible

This content is for reference only and cannot replace a professional veterinary diagnosis. If your pet shows signs of illness, please seek veterinary care promptly.