Warfarin sodium

What is this medication for?

• Long-term oral antithrombotic (venous thrombosis/PTE maintenance, ATE prevention historically) — controversial and largely displaced by LMWHs/clopidogrel (FAT CAT era)

How is it given?

Give exactly as your veterinarian prescribed — never change the dose or stop on your own unless your veterinarian has told you to.

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• Hemorrhage (dose-related; lag-masked) (The effect and the danger)
• Cumulative: 1–5 mg/kg × 5–15 d (dog); 1 mg/kg × 7 d (cat) (The rodenticide-adjacent math)

Worth a phone call to your veterinarian

• Single-dose toxicity: 5–50 mg/kg (Acute ingestion)

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Trouble breathing, collapse, seizures, severe vomiting or diarrhea, facial swelling, or pale gums

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: Active/preexisting bleeding, pregnancy, eye/CNS surgery planning, blood dyscrasias, uncontrolled high blood pressure, hepatic insufficiency, aneurysm, pericardial effusion
• has or has had: The 2–5 day lag works both directions — hold monitoring through changes

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • The longest interaction list in medicine: NSAIDs/aspirin (bleeding), metronidazole/trimethoprim-sulfa/chloramphenicol (↑ warfarin), phenobarbital/rifampin (↓ warfarin), thyroid hormone (↑ effect), vitamin K in diets (variable ↓)

Storage and handling

Tablet splitting precision at tiny animal doses; INR-friendly monitoring access required.

Good to know

• The mirror-image of the rodenticide file: warfarin IS long-acting-rodenticide pharmacology, dosed intentionally — and its rescue (vitamin K1 + plasma) is the same protocol in reverse.
• Why it lost: unproven mortality benefit + monitoring expense + bleeding risk + the FAT CAT trial (warfarin no better than aspirin in feline ATE, more bleeding) → LMWH/clopidogrel era.
• The initiation paradox (protein C falls before IIa) explains the heparin bridge — warfarin briefly THICKENS the blood.