Iron Tablets and Cats: The 20 mg/kg Line and 3 Phases

Elemental iron at 20 mg/kg is the line where a cat moves from stomach upset to a real poisoning, and one ordinary 325 mg ferrous sulfate tablet carries 65 mg of elemental iron — 16 mg/kg for a 4 kg cat, 80% of the way there in a single tablet. Two tablets pass the line. The 6 hours after ingestion are the window when the gut signs appear and when decontamination still works; the danger is that the cat often looks better at hour 8 and worse at hour 30.

What 20 mg/kg actually counts

The number is elemental iron, not the weight on the label. A 325 mg ferrous sulfate tablet is only 20% iron, so it delivers 65 mg; ferrous fumarate at 324 mg is 33% and delivers 106 mg; ferrous gluconate at 240 mg is 12% and delivers 27 mg. That gap is why the bottle matters more than the pill count. For a 4 kg cat, 20 mg/kg is 80 mg of elemental iron: 1.2 ferrous sulfate tablets, 0.75 of a ferrous fumarate tablet, or 3 ferrous gluconate tablets.

Product	Elemental iron per tablet	Tablets to reach 20 mg/kg in a 4 kg cat	Gut signs inside 6 h
Ferrous sulfate 325 mg	65 mg	1.2 tablets	Vomiting in 4–6 h at this dose
Ferrous fumarate 324 mg	106 mg	0.75 tablet	Vomiting, dark stool in 2–6 h
Ferrous gluconate 240 mg	27 mg	3 tablets	Mild nausea, drooling at 6 h
Carbonyl iron 65 mg	65 mg	1.2 capsules	Slower release, signs at 6–12 h
Prenatal vitamin, 27 mg	27 mg	3 tablets	Drooling, 1–3 vomits by 6 h

Why iron burns more than the milligram suggests

Iron is corrosive in the stomach before it is toxic in the blood. Unbound ferrous iron strips electrons from the mucosal lining, so the first damage is a chemical burn from the mouth to the small intestine: vomiting, dark coffee-ground material, abdominal pain. What is absorbed then circulates as free iron, which a cat has almost no spare binding capacity for; it dilates blood vessels, drops blood pressure, and pushes the blood acidic. The liver takes the next hit, and liver failure is what kills cats on day 2 to day 4, not the vomiting on day 1.

Size makes the arithmetic brutal. A 4 kg cat eating 2 tablets of ferrous sulfate takes 130 mg, which is 32.5 mg/kg — the moderate band, where systemic signs become possible. Five tablets is 325 mg, or 81 mg/kg, deep in the severe band. A 3 kg cat reaches 43 mg/kg on the same 2 tablets.

Three phases, three different pictures

Phase 1 runs 0 to 6 hours and is all gut: vomiting, drooling, diarrhea, a tender abdomen, and sometimes blood in the vomit. Phase 2 runs 6 to 24 hours and is the trap — the vomiting settles, the cat looks brighter, and free iron is quietly dropping the blood pressure and shifting the acid-base balance. Phase 3 runs 24 to 96 hours and is hepatic: jaundice, bleeding, collapse, and a cat that was sent home on day 1.

That is the whole reason a 6-hour observation at home is not enough for a large ingestion. The cat that is comfortable at hour 8 is the classic case that crashes at hour 30, and the only way to catch it is a blood panel at 24 hours, not a look at the cat.

The 6-hour window, and what to do inside it

Gut signs from iron appear inside 6 hours in most cats that took a meaningful dose, and iron tablets are radiopaque, so an X-ray taken in that window shows exactly how many are still in the stomach. If the cat vomited once and the dose was under 20 mg/kg, the risk of the systemic phases is low and the plan can be a phone call plus a 24-hour watch. If the dose was over 60 mg/kg, or the cat is vomiting repeatedly, the plan is a vet visit the same day regardless of how well the cat looks.

At home, remove the bottle and count what is missing, note the mg of elemental iron per tablet, the number taken and the cat's weight, and write the ingestion time. Do not feed the cat and do not push milk, antacids or food to bind the iron; none of that is reliable, and food in the stomach makes vomiting and endoscopy harder. Offer small sips of water only. Do not induce vomiting unless a vet directs it, because a cat that is already vomiting or dull should not be made to vomit again.

What the vet will do

Expect an abdominal X-ray to count remaining tablets, blood work for iron level, packed cell volume, glucose, liver enzymes and acid-base status, and intravenous fluids to protect the kidneys and the circulation. For significant exposures a vet may use deferoxamine, an iron chelator that binds free iron and turns the urine a vin rose color; it is a prescription drug given under veterinary supervision, and it is the reason early presentation changes the outcome. Activated charcoal binds iron poorly, so it is not the answer here. Liver and kidney values are typically rechecked at 24, 48 and 72 hours, and a cat that needed chelation often stays 2 to 4 days.

Which bottles cause this

• Prenatal vitamins: 27 to 65 mg of iron per tablet, the most common source in a house with a cat.
• Ferrous sulfate 325 mg: 65 mg per tablet, sold in bottles of 100 to 365.
• Iron plus vitamin C gummies: chewy, sugar-coated, and eaten as food rather than as a pill.
• Liquid iron supplements for children: 15 to 45 mg per 5 ml, and a spilled 30 ml is 90 to 270 mg.
• Dog supplements: 10 to 25 mg per tablet, harmless to a 30 kg dog and not to a 4 kg cat.

Prevention that actually holds

Keep every iron product in 1 lidded box at 150 cm or higher, in the original bottle with the label on, and count tablets at every refill. Prenatal vitamins belong in a zip bag inside a closed drawer, not in a daily pill organizer on the counter at 90 cm. If a tablet hits the floor, it is a 16 mg/kg dose for a 4 kg cat, so sweep, do not scan. Guests with supplements should put their bag in a closed room, and any chewable gummy product should be treated as food and kept in the fridge, not in a bowl.

If the tablet is unknown or the bottle is unlabelled, run the imprint through the toxin lookup before you drive, and take the bottle with you. Vomiting and drooling have a long list of causes; the plant version is covered in the houseplant emergency list. A cat that comes home after 2 days of fluids and blood work needs 48 hours of quiet monitoring, and the routine in post-catheter aftercare covers the basics.

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