NAVLE Pharmacology High-Yield Guide: Drugs, Mechanisms, Species

Pharmacology on the NAVLE is rarely a question about a dose. It is a question about a mechanism, a species that cannot tolerate a drug, a reversal agent, a food-animal rule, or a monitoring test. This guide collects the drug facts that reappear across every species block, organized by drug class, with the "gotcha" that the distractors are built around. Figures are taken from the Merck Veterinary Manual; where a number could not be verified there, it has been omitted rather than guessed.

Why Pharmacology Is Worth Dedicated Study

Pharmacology is not a separate section on the NAVLE; it is embedded in the canine, feline, equine, bovine, porcine, small ruminant, avian and exotic questions. The same fact (fluoroquinolones and feline retinas, ivermectin and herding breeds, penicillin and guinea pigs, phenylbutazone and dairy cows) can be asked in several species blocks. That makes drug knowledge unusually efficient to study: one list covers many questions. It also punishes the student who knows the drug but not the species exception, because the exam writers know exactly where those exceptions are. For how the species blocks are weighted, see the NAVLE complete guide.

Antimicrobials

Beta-Lactams (Penicillins, Cephalosporins)

• Mechanism: Inhibit cell wall synthesis; time-dependent killing, so dosing interval matters more than peak.
• Classic facts: Penicillin G for clostridial disease, listeriosis, Streptococcus equi and anthrax; clavulanate adds staphylococcal cover; ceftiofur is the labeled food-animal cephalosporin.
• Gotcha: Merck lists guinea pigs, chinchillas, birds, snakes and turtles as sensitive to procaine penicillin. The broader exam concept is that hindgut-fermenting small herbivores (guinea pigs, hamsters, rabbits, chinchillas) develop fatal antibiotic-associated enterocolitis from clostridial overgrowth when given penicillins, oral cephalosporins, lincosamides and macrolides. Mycoplasma has no cell wall and never responds to a beta-lactam. Most extralabel cephalosporin use in cattle, swine, chickens and turkeys is prohibited (cephapirin excepted); label-dose use for an unlabeled indication is the main exception.

Fluoroquinolones (Enrofloxacin, Marbofloxacin, Pradofloxacin, Orbifloxacin)

• Mechanism: Inhibit bacterial topoisomerases (DNA gyrase and topoisomerase IV); concentration-dependent, so once-daily high doses are appropriate. Excellent gram-negative and intracellular coverage; poor against anaerobes.
• Gotcha 1 (cats): Enrofloxacin doses above 5 mg/kg per day are associated with acute, irreversible retinal degeneration in cats, linked to a feline-specific defect in the ABCG2 transporter. Pradofloxacin appears least retinotoxic, then marbofloxacin and orbifloxacin.
• Gotcha 2 (young animals): Cartilage erosions in growing dogs and foals with high or prolonged dosing; avoid in the rapid growth phase.
• Gotcha 3 (food animals): Extralabel use of fluoroquinolones in food-producing animals is prohibited in the US. Enrofloxacin is labeled for specific indications in cattle and swine; use it exactly as labeled or not at all.

Aminoglycosides (Gentamicin, Amikacin, Neomycin)

• Mechanism: Bind the 30S ribosomal subunit; concentration-dependent with a post-antibiotic effect, which is why once-daily dosing is safer and equally effective. Need oxygen for uptake, so they are useless against anaerobes and in abscesses.
• Toxicity: Nephrotoxicity (proximal tubular necrosis; monitor urine for casts, keep patients hydrated, avoid concurrent NSAIDs and furosemide), ototoxicity (irreversible) and neuromuscular blockade, especially with anesthetics.
• Gotcha: Gentamicin persists in kidney tissue for many months in cattle (Merck cites 18 to 20 months in adults); it is effectively avoided in food animals and any use requires a FARAD withdrawal. Amikacin plus penicillin is the standard first-line combination in septic foals.

Tetracyclines (Oxytetracycline, Doxycycline, Minocycline)

• Mechanism: 30S ribosome, bacteriostatic. Drugs of choice for rickettsial and related organisms (Ehrlichia, Anaplasma, Rickettsia), Lyme disease, leptospirosis (doxycycline), chlamydia and mycoplasma.
• Gotcha: Chelate calcium in developing teeth and bone: hypoplastic enamel and yellow-brown discoloration in young animals and in the fetus. Doxycycline dry-pilled to cats causes esophageal erosion and stricture; follow every tablet with about 5 mL of water or use a liquid. Rapid IV oxytetracycline in cattle causes collapse (calcium chelation); expired tetracyclines cause tubular nephrosis; tetracyclines can cause severe, even fatal, diarrhea in stressed horses. Absorption is impaired by milk and antacids except for the lipid-soluble doxycycline and minocycline.

Chloramphenicol and Florfenicol

• Mechanism: 50S ribosome, bacteriostatic, broad spectrum including anaerobes, crosses the blood-brain barrier and into the eye and prostate.
• Gotcha: Chloramphenicol is prohibited in food animals because residues can cause irreversible, dose-independent aplastic anemia in people. Cats have limited glucuronidation, so the half-life is much longer (Merck: about 5.1 hours in cats versus under an hour in ponies) and dose-dependent bone marrow suppression is more likely. Florfenicol is the food-animal phenicol and does not carry the aplastic anemia risk.

Macrolides, Lincosamides and Others

• Macrolides: Erythromycin plus rifampin is the classic Rhodococcus equi combination in foals (treated foals can overheat; the mare can develop fatal colitis). Tilmicosin is cardiotoxic to humans and horses; never IV, never in horses.
• Lincosamides: Clindamycin for anaerobes and toxoplasmosis; lincomycin is deadly to horses and rabbits.
• Metronidazole: Anaerobes and Giardia; vestibular neurotoxicity at high cumulative doses; prohibited in food animals.
• Potentiated sulfonamides: Keratoconjunctivitis sicca and immune reactions in dogs (Dobermans over-represented), crystalluria, hypothyroidism with long courses; fatal arrhythmias reported in horses given IV trimethoprim-sulfa with detomidine.

Extralabel Rules and Prohibited Drugs

Extralabel use in food animals under AMDUCA requires a veterinarian, a valid VCPR, a therapeutic purpose, no suitable labeled alternative, no extralabel use in or on medicated feed (extralabel use in drinking water is permitted), records, and an extended withdrawal that prevents any violative residue. Merck's prohibited list: chloramphenicol; clenbuterol; diethylstilbestrol; dimetridazole, ipronidazole and other nitroimidazoles; furazolidone and nitrofurazone; sulfonamides in lactating dairy cattle (limited exceptions); fluoroquinolones; glycopeptides such as vancomycin; phenylbutazone in female dairy cattle 20 months and older; cephalosporins other than cephapirin in cattle, swine, chickens or turkeys, where the 2012 order bars unapproved doses, routes, frequencies or durations, human cephalosporin products and use for disease prevention, while label-dose use for an unlabeled indication remains allowed; and adamantanes or neuraminidase inhibitors in chickens, turkeys and ducks.