Toxicology shows up on roughly 5–8% of the NAVLE. That is 18–28 questions out of 360. Most of it clusters around 15–20 toxins that rotate through exams year after year. Know these cold and you are not leaving points on the table.
How Toxicology Appears on the NAVLE
The exam hits toxicology in three ways. First: a dog or cat presents with clinical signs and you have to name the toxin. Second: the toxin is named and you pick the antidote or explain the mechanism. Third: species-specific susceptibility—which animal cannot handle this exposure. Each type requires a different knowledge layer, and the fastest way to get all three right is to organize your learning by mechanism rather than by toxin name.
Top Toxins by Mechanism
Category 1: Cholinesterase Inhibitors — Organophosphates & Carbamates
These are the classic farm and insecticide toxins. Organophosphates (chlorpyrifos, malathion, diazinon) and carbamates (carbaryl, methomyl) both inhibit acetylcholinesterase, letting acetylcholine pile up at every synapse it touches. The clinical result is the SLUD syndrome: salivation, lacrimation, urination, defecation. Add miosis, bradycardia, and bronchospasm on the muscarinic side; muscle fasciculations and weakness on the nicotinic side.
OP / Carbamate
Insecticide
→
Inhibits AChE →
excess ACh at synapses
→
SLUD + miosis + bradycardia
Atropine + 2-PAM
(OP only)
Classic NAVLE Trap2-PAM (pralidoxime) works for organophosphate toxicity because it reactivates AChE before it “ages.” It does NOT work for carbamate toxicity—carbamate binding is spontaneously reversible, and 2-PAM can actually worsen carbamate toxicosis. The question will give you one of these by name; you need to recognize which category it falls in.
Category 2: Rodenticides
Three rodenticide classes appear repeatedly. Anticoagulants are the most common toxicosis in dogs. Bromethalin is the neurological curveball. Zinc phosphide is the exotic large-scale exposure question.
Brodifacoum / Bromadiolone
Anticoagulant rodenticide
→
Blocks Vit K epoxide reductase → depletes factors II, VII, IX, X
→
Hemorrhage / coagulopathy
Vitamin K1 × 4–6 wks
Bromethalin
Neurotoxic rodenticide
→
Uncouples oxidative phosphorylation in CNS → cerebral edema
→
Tremors, seizures, paralysis
No antidote—supportive only
NAVLE PearlAnticoagulant rodenticide treatment is Vitamin K1 (phytonadione) for a minimum of 4–6 weeks—NOT Vitamin K3. Signs are delayed 3–5 days post-ingestion because clotting factors must first be depleted. Factor VII has the shortest half-life, so PT prolongs before PTT. That sequence is a board-favorite detail.
Category 3: Nephrotoxins
Ethylene glycol
Antifreeze
→
Metabolized to oxalic acid → calcium oxalate monohydrate crystals in tubules
→
Acute kidney injury
Fomepizole (dogs)
Ethanol (cats)
Grapes and raisins in dogs cause AKI by an unknown mechanism—the threshold dose is not established, so any ingestion is treated as potentially toxic. Lilies in cats (Easter lily, Tiger lily, Asiatic lily, Stargazer) cause fatal renal tubular necrosis. The exam distinguishes true lilies (nephrotoxic) from peace lilies and calla lilies (oxalate irritants, not nephrotoxic). Any cat that has chewed on a true lily is an emergency regardless of the amount consumed.
Classic NAVLE TrapPeace lily and calla lily are NOT true lilies and do NOT cause renal failure. They contain calcium oxalate crystals that irritate the mouth and GI tract. When the question says “lily,” look for the genus: Lilium and Hemerocallis (daylilies) are nephrotoxic in cats. Spathiphyllum (peace lily) is not.
Category 4: Hepatotoxins
Xylitol
Sugar substitute (dogs only)
→
Triggers massive insulin release → hypoglycemia; high doses → hepatic necrosis
→
Hypoglycemia → liver failure
No antidote; dextrose + supportive
Acetaminophen
Cats >> dogs
→
NAPQI metabolite accumulates (cats lack glucuronidation) → oxidizes Hgb to MetHgb
→
Methemoglobinemia + hepatotoxicity
N-acetylcysteine + ascorbic acid
Blue-green algae (cyanobacteria, especially Microcystis) produces microcystins that cause acute, fulminant liver failure. Dogs that swim in or drink from scummy-looking ponds in summer are at risk. There is no antidote. Supportive care is all you have, and survival is rare with high-level exposures.
Category 5: CNS Toxins
Lead
Paint chips, old plumbing
→
Interferes with heme synthesis & nerve conduction → multi-system toxicity
→
GI signs + CNS (seizures, ataxia)
CaEDTA or DMSA (succimer)
Metaldehyde (slug bait) causes progressive muscle tremors and hyperthermia with no specific antidote—treatment is decontamination and methocarbamol for muscle relaxation. Marijuana/THC causes CNS depression in dogs: ataxia, dilated pupils, urinary dribbling, bradycardia. Signs look dramatic but are rarely fatal with supportive care.
Category 6: Chocolate & Methylxanthines
The mechanism is methylxanthine toxicity—theobromine is the primary culprit in dogs, with caffeine adding to the effect. Methylxanthines inhibit phosphodiesterase and block adenosine receptors, producing cardiac arrhythmias, tachycardia, tremors, seizures, and hyperthermia. The toxic threshold is approximately 20 mg/kg theobromine for mild signs; cardiac arrhythmias appear above 40–60 mg/kg.
Toxicity ranked most to least dangerous by chocolate type: baking chocolate (~400 mg theobromine per ounce) > dark chocolate (~130–160 mg/oz) > milk chocolate (~44–58 mg/oz) > white chocolate (negligible theobromine). A 10 kg dog eating 30g of baking chocolate is a genuine emergency. The same dog eating 30g of milk chocolate is not.
Master Antidote Quick-Reference Table
| Toxin | Mechanism | Key Clinical Signs | Antidote / Treatment |
|---|
| Organophosphate | Irreversible AChE inhibition | SLUD, miosis, bradycardia, fasciculations | Atropine + 2-PAM |
| Carbamate | Reversible AChE inhibition | SLUD, miosis, bradycardia | Atropine ONLY (NO 2-PAM) |
| Anticoagulant rodenticide | Blocks Vit K epoxide reductase | Hemorrhage, prolonged PT then PTT | Vitamin K1 × 4–6 weeks |
| Ethylene glycol (dog) | Oxalate crystals in renal tubules | Early CNS depression, later oliguria/AKI, Ca oxalate crystalluria | Fomepizole (4-MP) |
| Ethylene glycol (cat) | Same; cats more sensitive, faster AKI | Rapid AKI within 12–24 hrs, Ca oxalate crystalluria | Ethanol IV (fomepizole less effective in cats) |
| Acetaminophen | NAPQI → MetHgb; cats lack glucuronidation | Chocolate-brown blood, cyanosis, facial edema (cats) | N-acetylcysteine + ascorbic acid |
| Lead | Heavy metal → heme + nerve disruption | GI signs, seizures, nucleated RBCs on smear | CaEDTA or DMSA (succimer) |
| Nitrates (ruminants) | Ruminal conversion to nitrite → MetHgb | Brown mucous membranes, dyspnea, death | Methylene blue 4–8 mg/kg IV slowly |
| Opioid overdose | Mu-receptor agonism | CNS/respiratory depression, miosis, bradycardia | Naloxone 0.01–0.02 mg/kg IV/IM/SQ |
| Pit viper envenomation | Hemotoxic venom → coagulopathy, cytotoxicity | Local swelling, coagulopathy, shock | Antivenin (crotalidae); supportive |
Species-Specific Susceptibilities
Species-specific questions test whether you know the metabolic difference that makes a toxin dangerous in one species but not another. These are not trivia—they have a mechanistic reason.
| Species | Unique Susceptibility | Reason |
|---|
| Cats | Acetaminophen, permethrin, true lilies, aspirin | Deficient glucuronidation (UGT1A6 missing); permethrin—lack CYP450 pyrethrins metabolism |
| Dogs | Xylitol, grapes/raisins, macadamia nuts | Species-specific insulin dysregulation (xylitol); mechanism unknown for grapes and macadamia |
| Horses | Ionophores (monensin, lasalocid) | Disrupts Na+/K+ transport in cardiac muscle; doses safe for poultry are lethal in horses |
| Ruminants | Nitrates (lush forage, drought-stressed plants) | Ruminal bacteria reduce nitrates to nitrites → MetHgb; monogastrics less susceptible |
| Birds | Avocado (Persin toxin), PTFE/Teflon fumes | Persin causes cardiac failure; PTFE fumes cause acute pulmonary hemorrhage at temperatures safe for humans |
NAVLE PearlPermethrin spot-on flea treatments designed for dogs kill cats. The cat lacks the CYP450 enzymes to metabolize pyrethroids. Clinical signs are dramatic: violent muscle tremors, hypersalivation, and seizures. Treatment is aggressive bathing to remove residue, methocarbamol for tremors, and IV fluids. The board question often frames it as a cat owner using a dog product on their cat—recognize the scenario.
NAVLE TipWhen a NAVLE toxicology question gives you a signalment, read it carefully. Species, age, and environment are all clues. A kitten from a multi-pet household: suspect permethrin or spot-on product misuse. A farm dog: think organophosphate. A show horse in a barn that recently changed feed: think ionophores (contaminated feed). The scenario is always pointing you somewhere if you slow down and read it.