Pharmacokinetics and Pharmacodynamics: How Drugs Work in the Animal Body

What Is Pharmacokinetics?

Pharmacokinetics describes what the animal body does to a drug. It is organized into four sequential phases, often abbreviated ADME: Absorption, Distribution, Metabolism, and Excretion. Understanding each phase helps veterinary technicians and assistants anticipate how a drug will behave, why doses differ between species, and when a patient may be at risk for toxicity or treatment failure.

Absorption

Absorption is the movement of a drug from its administration site into the bloodstream. The route of administration strongly influences how quickly and completely this happens. Intravenous (IV) drugs bypass absorption entirely and act fastest. Oral drugs must survive the gastrointestinal environment before reaching systemic circulation.

Species differences matter significantly here. Horses have a large hindgut populated with billions of microorganisms. Oral antibiotics that disturb this microbial population can cause life-threatening colitis, which is why many drugs safe in small animals are avoided or used with extreme caution orally in horses. Cats have a smaller stomach volume and may vomit oral medications more readily than dogs. Bioavailability — the fraction of a dose that actually reaches the bloodstream unchanged — varies by drug, formulation, and species.

Distribution

Once in the bloodstream, a drug distributes into tissues. The volume of distribution (Vd) describes how widely a drug spreads throughout the body. A high Vd means the drug penetrates tissues deeply (useful for infections inside cells or the central nervous system); a low Vd means it stays mostly in the plasma.

Protein binding also affects distribution. Many drugs bind reversibly to plasma proteins such as albumin. Only the free (unbound) fraction is pharmacologically active. If a second drug displaces the first from protein binding sites, the free concentration of the first drug rises suddenly, potentially causing toxicity — a critical drug-drug interaction mechanism.

Body composition influences distribution too. Obese patients have more adipose tissue, which can accumulate lipid-soluble drugs. Neonatal and geriatric patients have different protein levels and body water percentages, shifting distribution behavior.

Metabolism (Biotransformation)

Metabolism converts drugs — usually in the liver — into metabolites that are often more water-soluble and easier to excrete. The cytochrome P450 enzyme system carries out many of these reactions. Phase I reactions (oxidation, reduction, hydrolysis) alter the drug chemically; Phase II reactions (conjugation) attach molecules such as glucuronide or sulfate to make the metabolite more polar.

Cats have a well-known deficiency in glucuronidation, one of the key Phase II conjugation pathways. Drugs that rely heavily on glucuronidation — including acetaminophen and certain NSAIDs — accumulate to toxic levels in cats at doses that are safe in dogs or humans. This is a critical patient safety point: acetaminophen is potentially fatal in cats and must never be administered to them. Always verify species-specific safety before any medication is given.

Dogs metabolize some drugs more slowly than humans but faster than cats for certain compounds. Horses rely heavily on hepatic metabolism; liver disease in any species reduces metabolic capacity and prolongs drug action.

Excretion

Most drugs and their metabolites leave the body through the kidneys in urine, although bile, feces, expired air, and milk are also routes. Renal excretion depends on glomerular filtration rate (GFR). Patients with kidney disease excrete drugs more slowly, so drug levels can accumulate between doses. Dose adjustments or extended dosing intervals are often necessary in animals with renal insufficiency — always flag this to the supervising veterinarian.

The half-life (t½) is the time it takes for the plasma concentration of a drug to fall by half. After approximately five half-lives, a drug is considered effectively cleared from the body (less than about 3% of the original concentration remains).

Pharmacodynamics: How Drugs Act

While pharmacokinetics is what the body does to the drug, pharmacodynamics is what the drug does to the body. Most drugs work by binding to specific receptors — proteins on cell surfaces or inside cells. The degree of effect depends on how well a drug binds (affinity) and how strongly it activates the receptor (efficacy).

• An agonist binds and activates a receptor, producing a response (e.g., opioids binding mu-receptors to reduce pain).
• An antagonist binds but does not activate, blocking the receptor (e.g., naloxone reversing opioid effects).

The therapeutic window is the concentration range between the minimum effective concentration (MEC) and the minimum toxic concentration (MTC). Maintaining drug levels within this window is the practical goal of dosing.

Drug-Drug Interactions

Polypharmacy — using multiple drugs simultaneously — is common in veterinary patients. Interactions can be pharmacokinetic (one drug alters the absorption, distribution, metabolism, or excretion of another) or pharmacodynamic (combined effects at the receptor level). Always consult Plumb's Veterinary Drug Handbook or another current reference when a patient is on multiple medications, and report potential interactions to the veterinarian before administration.

Safety Summary

Scope of practice is paramount. Veterinary assistants and technicians do not prescribe medications; they administer them under veterinary supervision and monitor patients for adverse effects. Species differences, organ function, age, and concurrent medications all influence drug response. When in doubt, stop and verify with the veterinarian.