Bromethalin toxicosis in dogs and cats: dose calculation, decontamination and prognosis

Bottom line

No antidote exists for bromethalin poisoning, and the key to successful treatment is early and aggressive decontamination [1]. The decision is therefore arithmetic — convert the worst-case ingestion into mg/kg of bromethalin active ingredient, not grams of bait, and decontaminate from 0.1 mg/kg in dogs and 0.05 mg/kg in cats [2]. Cats are far more sensitive, with a reported oral LD50 of 0.4-0.71 mg/kg against 2.38-5.6 mg/kg in dogs [1]. Signs are delayed, so a normal neurologic examination in the first hours is not reassurance: neurological signs may appear anywhere within 1-7 days of exposure [1].

Why you are seeing bromethalin

The US EPA's 2008 risk mitigation decision removed the second-generation anticoagulants — brodifacoum, bromadiolone, difenacoum and difethialone — from the consumer market; consumer bait stations now contain bromethalin, chlorophacinone or diphacinone [3]. A California laboratory series of 223 cases (123 dogs, 100 cats) recorded a 2.8-fold rise, from 59 cases in 2010-2016 to 164 in 2017-2023 [4]. Because the name resembles bromadiolone and brodifacoum, bromethalin is often mistaken for an anticoagulant [5]: vitamin K1 does nothing, and a normal PT is not reassurance.

Pathophysiology and toxicokinetics

Bromethalin is a pro-toxicant, bioactivated by hepatic N-demethylation to desmethylbromethalin, which uncouples oxidative phosphorylation in CNS mitochondria [1]. ATP falls, Na+/K+-ATPase pumps fail, cells lose osmotic control, and fluid accumulates within myelin sheaths — white-matter spongiosis, cerebral and spinal cord oedema, raised CSF pressure [5]. Absorption is fast (plasma concentrations in rats peak at about four hours); elimination is slow and biliary, with a rat plasma half-life of about six days and enterohepatic recirculation suspected [5]. The half-life has not been established in dogs and cats, though it may resemble the rat value [1]. That biliary loop is the entire rationale for repeat-dose charcoal.

Metabolite figures are not parent-compound figures. In vitro, desmethylbromethalin is about two to three times more potent an uncoupler than bromethalin, and the guinea pig — which does not perform effective N-demethylation — has an oral LD50 for bromethalin of more than 1 g/kg but an oral LD50 for desmethylbromethalin of 7.5 mg/kg [5]. Every canine and feline figure below is for the parent compound as ingested.

Dose arithmetic: bait grams are not bromethalin milligrams

Most bromethalin baits are 0.01%, some commercial products are more concentrated, and only the packaging is a reliable source of the concentration [2]; Merck cites 0.01% to 0.025%, i.e. 0.1-0.25 mg of bromethalin per gram of bait [1]. At 0.01%, each gram of bait contains 0.1 mg of bromethalin [2] and one ounce contains 2.84 mg [5].

The published worked example: a 21 kg hound mix with a worst-case exposure of 6.75 ounces of 0.01% bait ingested 6.75 oz x 28.4 g/oz = 192 g of bait; 0.1 mg/g x 192 g = 19.2 mg of bromethalin; 19 mg / 21 kg = 0.9 mg/kg of bromethalin [2]. Nearly seven ounces of bait sounds catastrophic; 0.9 mg/kg is the number that selects the protocol. Compute from the missing bait, and if the amount cannot be established, treat at the most aggressive level [5].

All figures below are mg/kg of bromethalin active ingredient, oral route.

Figure	Dog	Cat
Oral LD50 (bait formulations)	2.38-5.6 mg/kg [5]	about 0.54 mg/kg [5]
Oral LD50 (range cited)	—	0.4-0.71 mg/kg [1]
Reported minimum lethal dose	2.5 mg/kg [5]	0.45 mg/kg [5]
Uniformly fatal dose	—	0.75 mg/kg [5]
Lowest dose producing signs, APCC records	0.15 mg/kg, hindlimb weakness, rare [2]	below 0.24 mg/kg [2]
Lowest reported fatal dose, APCC records	0.95-1.05 mg/kg [5]	0.45 mg/kg [2]
Begin decontamination at	0.1 mg/kg [2]	0.05 mg/kg [2]

An LD50 is a median lethal dose, not a threshold of harm: these are medians, and any individual patient may be more sensitive [2]. The experimental finding that doses below 1.5 mg/kg were not associated with clinical signs [5] does not make anything under 1.5 mg/kg safe — the same source records APCC deaths in dogs at 0.95-1.05 mg/kg [5], and the canine decontamination threshold sits an order of magnitude lower at 0.1 mg/kg [2]. Dogs under one year may be more susceptible, possibly from an immature blood-brain barrier [2].

Two syndromes, and why an asymptomatic patient is not reassuring

In dogs, presentation is dose-dependent. Doses at or above the average LD50 (approximately 3.54 mg/kg) may cause a convulsant syndrome within 4-36 hours: hyperexcitability, hyperthermia, tremors, and focal or generalised seizures that may be sound- or light-induced [1]. Doses below that average may produce a paralytic (subacute or chronic) syndrome, typically within 1-5 days [1], starting with hindlimb weakness and ataxia and progressing to depression, tremors, hindlimb paralysis, loss of deep pain, Schiff-Sherrington posture, seizures and coma [5]. Tourdot cites a canine LD50 of 3.65 mg/kg for the same split [2]; the averages differ slightly by source, and neither is a threshold.

Cats typically develop the paralytic syndrome irrespective of the dose ingested [1], and may add abdominal distension from ileus and inability to urinate with increased urethral tone, indicating upper motor neuron bladder paralysis [5]. A patient presented within four hours of eating bait will almost always look normal, and looking normal is not evidence of a small dose.

Decontamination is the treatment

Induce emesis if ingestion was recent — Merck frames the window as within 4 hours [1] — but not in symptomatic animals [5]. Then give charcoal, repeatedly: the first dose should be the largest, 1 to 2 g/kg, ideally with a cathartic such as sorbitol unless the patient already has diarrhoea, and subsequent doses half the original dose with no cathartic [2]. Merck states activated charcoal 1-2 g/kg PO as an aqueous slurry, repeated every 8 hours for up to two additional doses [1].

Species	Dose ingested (mg/kg bromethalin)	Under 4 h since exposure	Over 4 h since exposure
Dog	0.1-0.49	Emesis or 1 dose charcoal	1 dose charcoal
Dog	0.5-0.75	Emesis + charcoal q8h for 24 h (3 doses)	Charcoal q8h for 24 h (3 doses)
Dog	>0.75	Emesis + charcoal q8h for 48 h (6 doses)	Charcoal q8h for 48 h (6 doses)
Cat	0.05-0.09	Emesis or 1 dose charcoal	1 dose charcoal
Cat	0.1-0.3	Emesis + charcoal q8h for 24 h (3 doses)	Charcoal q8h for 24 h (3 doses)
Cat	>0.3	Emesis + charcoal q8h for 48 h (6 doses)	Charcoal q8h for 48 h (6 doses)

Table content follows the ASPCA APCC recommendations as published by Tourdot [2]. If there is no evidence of charcoal in the rectum within 8 hours after the first dose, consider administering a warm water enema; separately, if the faeces are the colour of the bait, a gentle warm water enema at 5-10 mL/kg is recommended to help decontaminate the colon [2]. Maintain IV balanced crystalloids with unlimited access to oral water throughout [2]. Intravenous lipid emulsion and cholestyramine are mechanistically plausible and unproven: no published evidence supports their clinical use [2].

Iatrogenic hypernatremia

The potential for activated charcoal to cause potentially lethal hypernatremia should not be underestimated; it is thought to follow charcoal's osmotic effect drawing water into the gut, and its signs can resemble bromethalin toxicosis itself [2]. Take a baseline serum sodium before the first dose and recheck before each subsequent dose; if sodium increases 5 mmol/L or more from baseline, provide free water (oral, enema, or 5% dextrose in water) and postpone the following dose until sodium has decreased [2]. In the published case, sodium rose from 146 mmol/L to 151 mmol/L seven hours after charcoal with sorbitol, corrected with increased fluids and an enema, and the remaining doses were given without sorbitol [2].

A 2025 multi-centre retrospective bounds how often this happens, but not in this scenario. Among 97 dogs treated for acute toxicant ingestion of any kind with multi-dose activated charcoal, with or without sorbitol, none developed hypernatremia (defined as serum sodium above 155 mEq/L), median sodium fell from 146.7 mEq/L at presentation to 144 mEq/L at 24-48 hours, all 97 survived to discharge, and there was no statistically significant difference between the type of toxicant ingested and changes in serum sodium [6]. That is an all-comers toxicant cohort, not a bromethalin cohort, and because it pools cathartic with non-cathartic dosing it cannot isolate sorbitol — which is the axis Tourdot's case points at, since the rise followed sorbitol-containing charcoal and the sorbitol-free doses that followed were uneventful [2].

Nothing here retires the risk. Decontaminate the patient whose calculated dose warrants it, drop the cathartic after the first dose, and check the sodium — the monitoring is the cheap part and it is the part that is certain.

Diagnosis: what you can and cannot confirm

There are no pathognomonic clinicopathologic abnormalities in dogs or cats [5]. Confirmation depends on detecting bromethalin or its major metabolite in liver, kidney, brain or fat, but that analysis is available at only a limited number of veterinary diagnostic laboratories and is rarely performed, usually at necropsy [1]. In the California series, adipose tissue (37%), liver (20%) and brain (19%) were the commonest samples, and cats were significantly more likely than dogs to have confirmed desmethylbromethalin exposure (60% vs 25%) [4].

Antemortem confirmation is possible but not fast: three dogs were diagnosed antemortem by desmethylbromethalin in fat or serum, with concentrations tracking outcome (below 0.1 ng/g in a dog that recovered, above 280 ng/g in a fatal case), and MRI showed extensive T2W and T2W-FLAIR hyperintensity of cerebral and brainstem white matter without contrast enhancement [7]. Neither returns in time to guide decontamination. Differentials span rabies, metaldehyde, strychnine, zinc phosphide and ethylene glycol for the convulsant picture, and spinal cord trauma, neoplasia, intervertebral disc disease and idiopathic polyradiculoneuritis for the paralytic one [5]; add iatrogenic hypernatremia to both in a patient already receiving charcoal.

Symptomatic management and prognosis

Care is symptomatic and supportive: seizure control, nutritional support, and nursing care for recumbent animals [5]. Merck lists mannitol at 0.25-2 g/kg IV over 20-30 minutes, repeated every 4-8 hours, and methocarbamol at 55-220 mg/kg IV to effect or as a CRI at 10 mg/kg/h [1].

Be honest about what oedema-directed therapy achieves. In dogs, corticosteroids and osmotic agents lowered CSF pressure initially, but pressure returned to pretreatment levels quickly once treatment stopped and had little effect on progression; in cats no effect on onset or progression was seen, probably because fluid trapped within myelin sheaths is harder to mobilise [5]. Against that, a 9-year-old Norwich terrier with obtunded mentation, seizures, cranial nerve deficits and tetraparesis recovered completely after intravenous lipid emulsion, mannitol and ginkgo biloba, which the authors noted had not previously been reported after severe toxicosis [8]. One case does not establish efficacy, but severe canine cases are not uniformly hopeless.

Mild signs such as hindlimb weakness may resolve with time, while animals with paralysis or seizures generally have a poor to grave prognosis for survival; signs may progress for one or two weeks and take several weeks to resolve [5], rarely persisting in mild form with some permanency [1]. Experimentally, dogs given a single oral dose of bromethalin at 6.25 mg/kg developed hyperexcitability, tremors, seizures, depression and death within 15-63 hours [9].

Frequently Asked Questions

Is there an antidote for bromethalin? No. The Merck Veterinary Manual professional edition states that no antidote exists for bromethalin poisoning, and that the key to successful treatment is early and aggressive decontamination [1]; the ASPCA APCC toxicology brief likewise calls early, aggressive decontamination the best treatment for bromethalin ingestion [5]. Vitamin K1 is irrelevant, and a normal PT is not reassurance.

A dog ate bait 10 hours ago and is neurologically normal. Do I still decontaminate? Yes, if the calculated dose crosses the threshold. The Merck Veterinary Manual professional edition reports convulsant signs typically 4-36 hours after ingestion and paralytic signs typically within 1-5 days, with neurological signs possible anywhere within 1-7 days of exposure [1]. An asymptomatic patient in the first hours is simply still inside the latency.

How do I turn grams of bait into a mg/kg dose of bromethalin? Today's Veterinary Practice (Tourdot, ASPCA APCC) states that a bromethalin concentration of 0.01% equals 0.1 mg of bromethalin in each gram of bait [2], and the ASPCA APCC toxicology brief gives 2.84 mg per ounce of 0.01% bait [5]. Multiply grams of bait by the mg/g concentration read off the label, then divide by body weight in kg. Most products are 0.01%, but the Merck Veterinary Manual professional edition cites a range of 0.01% to 0.025%, or 0.1-0.25 mg/g [1].

At what dose do I start decontamination in a cat versus a dog? Today's Veterinary Practice, publishing the ASPCA APCC recommendations, puts the starting point at 0.05 mg/kg of bromethalin in cats and 0.1 mg/kg in dogs [2]. The ASPCA APCC toxicology brief reports a feline oral LD50 of about 0.54 mg/kg for bait formulations, a feline minimum lethal dose of 0.45 mg/kg and 0.75 mg/kg as uniformly fatal in cats, against a canine oral LD50 of 2.38-5.6 mg/kg [5].

Why repeated activated charcoal rather than a single dose? Because bromethalin and its metabolite appear to undergo enterohepatic recirculation, the ASPCA APCC toxicology brief notes that multiple doses have been suggested even hours after ingestion, and that this may delay onset and reduce the magnitude of signs in dogs, though it may be less beneficial in cats [5]. Today's Veterinary Practice advises the first dose be the largest at 1-2 g/kg, with subsequent doses at half that and no cathartic [2].

How dangerous is the hypernatremia risk from repeated charcoal? Take it seriously, but do not let it stop appropriate decontamination. Today's Veterinary Practice warns that activated charcoal can cause potentially lethal hypernatremia whose signs mimic bromethalin toxicosis, and advises a baseline serum sodium with a recheck before each dose, giving free water and postponing the next dose if sodium rises 5 mmol/L or more from baseline [2]; in the case it reports, the rise followed sorbitol-containing charcoal and the later sorbitol-free doses were uneventful [2]. A 2025 Frontiers in Veterinary Science retrospective by Gabriel and colleagues found that none of 97 dogs given multi-dose activated charcoal for acute toxicant ingestion of any kind, with or without sorbitol, developed hypernatremia above 155 mEq/L, and all survived to discharge [6] — but that is an all-comers toxicant cohort that pools cathartic with non-cathartic dosing, so it does not measure the sorbitol risk in a bromethalin patient. Decontaminate, drop the cathartic after the first dose, and monitor sodium.