Animal Pharmacology Calculators

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Animal pharmacology is the study of how drugs interact with the bodies of non-human animals — how they are absorbed, distributed, metabolized, and excreted, and what pharmacological effects they produce. Drug metabolism varies dramatically between species due to differences in liver enzyme activity, plasma protein binding, gut flora, and body composition. These species-specific differences mean that drug doses, dosing intervals, and safety profiles must be determined separately for each species — a drug safe and effective in humans may be highly toxic in cats, dogs, or birds.

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Pharmacokinetic Principles in Animals

The same ADME framework applies across species:

  • Absorption: Bioavailability varies with route, gastric pH, GI anatomy, and gut motility — all species-dependent
  • Distribution: Volume of distribution depends on body composition (fat %, plasma protein levels, membrane permeability)
  • Metabolism: Hepatic cytochrome P450 and phase II enzyme activities vary widely between species — the main source of species differences in drug safety and dosing
  • Excretion: Renal function (GFR) scales with body weight; bile excretion pathways differ

Species-Specific Metabolic Differences

  • Cats: Deficient glucuronyl transferase — acetaminophen, aspirin, many NSAIDs, and benzyl alcohol are highly toxic
  • Dogs: N-acetyltransferase deficiency — sulfonamide drugs may cause idiosyncratic reactions more frequently
  • Birds: Generally faster metabolic rates; many drugs require more frequent dosing; avocado (persin) is toxic
  • Horses: Hindgut fermentation — oral antibiotics that disrupt cecal flora can cause fatal colitis (e.g., oral penicillins)
  • Ruminants: Rumen microbiome affects oral drug absorption and can metabolize drugs before absorption

Veterinary Dose Calculation

Most veterinary drugs are dosed by body weight (mg/kg):

Dose (mg) = Dose rate (mg/kg) × Body weight (kg)

Volume to administer: Volume (mL) = Dose (mg) / Concentration (mg/mL)

Allometric scaling adjusts doses across very different body sizes: metabolic rate scales approximately as body weight^0.75, so dose per kg is not always constant across species.

Therapeutic Index

The therapeutic index (TI) = LD₅₀ / ED₅₀. Drugs with narrow TI require careful dosing and monitoring. Many veterinary anesthetics (barbiturates, α₂-agonists) have narrow TI; NSAIDs in cats have narrow TI due to reduced glucuronidation.

Glossary

ADME
Absorption, Distribution, Metabolism, Excretion — the four pharmacokinetic processes determining how a drug moves through the body. Species-specific differences in each process account for variation in drug dosing and safety across animal species.
Therapeutic Index (TI)
The ratio of the lethal dose (LD₅₀) to the effective dose (ED₅₀). A high TI indicates a wide safety margin; a low TI requires careful dosing. Used to assess drug safety in veterinary and human pharmacology.
Allometric Scaling
Adjustment of drug doses across species or body sizes based on metabolic rate, which scales approximately as body weight^0.75. Used when extrapolating pharmacokinetic parameters between species of very different sizes.

Frequently Asked Questions

Drug metabolism varies enormously between species due to differences in liver enzyme activities, plasma protein binding, renal function, body composition, and gut anatomy. A dose safe in humans may be rapidly metabolized and excreted in one species but accumulate to toxic levels in another due to absent or slower metabolic pathways. Doses must be determined empirically for each species, and even within species, age, body condition, and disease state significantly affect drug handling.

Most veterinary drugs are dosed in mg/kg body weight: Dose (mg) = dose rate (mg/kg) × body weight (kg). Volume to administer = Dose (mg) / drug concentration (mg/mL). Example: 10 mg/kg amoxicillin for a 5 kg cat = 50 mg; if the suspension is 50 mg/mL, give 1 mL. Allometric scaling (dose ∝ BW^0.75) is used when extrapolating across very different body sizes.

Ibuprofen and other NSAIDs inhibit cyclooxygenase (COX) enzymes in dogs as in humans, but dogs are more sensitive to NSAID-induced gastrointestinal ulceration, renal injury, and hepatotoxicity. Dogs also metabolize ibuprofen more slowly than humans — it has a longer half-life and can accumulate to toxic concentrations. Veterinary NSAIDs (meloxicam, carprofen) are formulated with better safety profiles for canine use and should always be used instead of human formulations.

The therapeutic index (TI) = LD₅₀/ED₅₀ — the ratio of lethal dose to effective dose. A high TI means a drug is relatively safe (wide margin between effective and toxic doses); a low TI means careful dosing is essential. Digoxin, aminoglycoside antibiotics, and many anesthetics have narrow TIs requiring therapeutic drug monitoring. Drugs with narrow TI are more dangerous in species with impaired metabolism (e.g., most NSAIDs in cats).