4.1 Introduction to Veterinary Drug Classes

4.1 Introduction to Veterinary Drug Classes

In veterinary medicine, the veterinary assistant plays a vital role in the pharmacy. Under the direct supervision of a licensed veterinarian or a credentialed veterinary technician, the assistant helps fill, package, label, and inventory medications. Understanding the fundamental classes of veterinary drugs, their indications, common side effects, and the distinction between active ingredients and trade names is essential for ensuring patient safety and providing accurate client education.

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Active Ingredient vs. Trade Name

Every medication has two primary names: its active ingredient (generic name) and its trade name (brand name).

• Active Ingredient (Generic Name): This is the official, chemically defined name of the active drug substance. It remains the same regardless of which pharmaceutical company manufactures it. For example, carprofen is the active ingredient in a widely used anti-inflammatory medication.
• Trade Name (Brand Name): This is the commercial name chosen by the manufacturer, registered as a trademark, and marketed under a specific brand. For example, Rimadyl and Novox are trade names for carprofen.

Generic versions of drugs contain the same active ingredients in the same concentration and dosage form as their brand-name counterparts, but they may differ in their inactive ingredients (excipients), such as binders, preservatives, flavorings, and colorings.

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Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)

Non-steroidal anti-inflammatory drugs (NSAIDs) are among the most frequently prescribed medications in small animal practice.

Mechanism of Action

NSAIDs work by inhibiting the cyclooxygenase (COX) enzymes, which are responsible for the synthesis of prostaglandins. Prostaglandins are chemical messengers that promote inflammation, pain, and fever (pyrexia). Most modern veterinary NSAIDs are selective for the COX-2 enzyme (which is induced during inflammation) while sparing the COX-1 enzyme (which protects the gastric mucosa and maintains renal blood flow).

Key Veterinary NSAIDs

• Carprofen (Rimadyl, Novox): Frequently prescribed for dogs for osteoarthritis and post-operative pain.
• Meloxicam (Metacam): Liquid suspension commonly used in dogs and labeled for a single one-time injection in cats (post-operative).
• Robenacoxib (Onsior): Available in tablets and injections specifically approved for short-term use in cats (up to 3 days).
• Deracoxib (Deramaxx) & Firocoxib (Previcox): Selective COX-2 inhibitors used for dogs.

Indications

NSAIDs are indicated for the control of pain and inflammation associated with osteoarthritis, orthopedic or soft-tissue surgery, and for the management of fever.

Common Side Effects & Toxicity

The most common side effects of NSAIDs affect the gastrointestinal (GI) tract, kidneys, and liver:

1. Gastrointestinal Upset: Vomiting, diarrhea, anorexia (loss of appetite), and lethargy.
2. GI Ulceration and Bleeding: Signs include dark, tarry stools (melena) or blood in the vomit.
3. Renal Toxicity: Decreased blood flow to the kidneys can lead to acute kidney injury, particularly in patients that are dehydrated, hypotensive, or have pre-existing kidney disease.
4. Hepatotoxicity: Rare, idiosyncratic liver damage (manifesting as elevated ALT/ALP liver enzymes, icterus, and vomiting).

[!WARNING] Safety Traps:

• Never combine NSAIDs with other NSAIDs or with Corticosteroids (e.g., prednisone, dexamethasone). Doing so significantly increases the risk of life-threatening gastrointestinal ulceration and perforation. A "washout period" of 3 to 7 days is required when switching between these medications.
• Human Medications: Ibuprofen (Advil), naproxen (Aleve), and acetaminophen (Tylenol) are highly toxic to dogs and cats. Acetaminophen is extremely lethal to cats because they lack the liver enzyme glucuronyl transferase to metabolize it, resulting in Heinz body anemia, methemoglobinemia (chocolate-colored blood/mucous membranes), facial swelling, and liver failure.

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Antibiotics (Antimicrobials)

Antibiotics are chemical substances used to treat bacterial infections. They are classified into families based on their chemical structure and mechanism of action.

Key Antibiotic Classes

1. Penicillins & Aminopenicillins:
   • Examples: Amoxicillin, Amoxicillin/Clavulanate (Clavamox - potentiated with clavulanic acid to overcome bacterial resistance).
   • Indications: Skin, respiratory, and urinary tract infections (UTIs).
   • Side Effects: Mild GI upset (diarrhea, vomiting). Allergic reactions (urticaria, anaphylaxis) can occur.
2. Cephalosporins:
   • Examples: Cephalexin (Rilexine), Cefovecin (Convenia - a long-acting injectable antibiotic that lasts for 14 days, highly convenient for difficult-to-pill patients).
   • Indications: Skin infections (pyoderma), soft tissue wounds, and cystitis.
3. Fluoroquinolones:
   • Examples: Enrofloxacin (Baytril), Marbofloxacin (Zeniquin), Orbifloxacin (Orbax).
   • Indications: Severe, deep infections (otitis, respiratory infections, genitourinary infections).
   • Side Effects: Contraindicated in growing puppies because they can cause articular cartilage damage in developing joints. In cats, high doses of enrofloxacin can cause acute retinal degeneration and irreversible blindness.
4. Tetracyclines:
   • Examples: Doxycycline, Minocycline.
   • Indications: Tick-borne diseases (Lyme disease, Ehrlichia, Anaplasma, Rocky Mountain spotted fever) and respiratory pathogens (e.g., Bordetella bronchiseptica).
   • Side Effects: GI upset, photosensitivity, and tooth discoloration in young animals.
   • Safety Note: In cats, dry-pilling doxycycline can cause esophageal strictures (narrowing of the esophagus due to acid-like irritation). Tablets or capsules must always be followed by at least 5-10 mL of water or given with food.
5. Aminoglycosides:
   • Examples: Gentamicin, Amikacin, Neomycin.
   • Indications: Severe Gram-negative infections.
   • Side Effects: High risk of nephrotoxicity (kidney damage) and ototoxicity (inner ear damage, leading to deafness and vestibular signs). Patients on systemic aminoglycosides must have their kidney values monitored closely.

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Antiparasitics

Antiparasitics target internal parasites (endoparasites) or external parasites (ectoparasites).

Ectoparasiticides

• Isoxazolines: Afoxolaner (NexGard), Fluralaner (Bravecto), Sarolaner (Simparica), Lotilaner (Credelio). These are oral monthly or quarterly chewables that kill fleas and ticks by interfering with their nervous system. They should be used with caution in dogs with a history of seizures or neurological disorders.
• Topical Fipronil/Imidacloprid (Frontline, Advantage): Spot-on treatments that spread through the skin's lipid layer to kill fleas and ticks.

Endoparasiticides (Anthelminthics)

• Macrocyclic Lactones: Ivermectin, Milbemycin oxime, Moxidectin, Selamectin. These are the active ingredients in most monthly heartworm preventatives (e.g., Heartgard, Interceptor, Advantage Multi). They prevent heartworm (Dirofilaria immitis) by killing microfilariae before they mature into adults.
  • MDR1 Genetic Mutation: Herding breeds (Collies, Australian Shepherds, Shetland Sheepdogs) may carry the MDR1 gene mutation, which impairs the blood-brain barrier. At standard heartworm preventative doses, these drugs are safe, but at higher doses (such as those used to treat mange), they can cause severe neurotoxicity (tremors, ataxia, blindness, death).
• Pyrantel Pamoate (Nemex, Strongid): Commonly used dewormer that treats roundworms (ascarids) and hookworms.
• Fenbendazole (Panacur): A broad-spectrum dewormer effective against roundworms, hookworms, whipworms, and the protozoan Giardia. Given for consecutive days.
• Praziquantel (Droncit): Specifically targets tapeworms (cestodes) acquired from ingesting fleas or raw prey.

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Antiemetics (Anti-nausea & Anti-vomiting)

Antiemetics control vomiting and nausea by blocking neurotransmitters in the brain or peripheral nervous system.

• Maropitant Citrate (Cerenia): An NK1 receptor antagonist that acts centrally in the emetic center of the brain and peripherally in the gastrointestinal tract. It is highly effective for acute vomiting and motion sickness.
  • Clinical Tip: Injectable Cerenia can cause stinging or pain at the injection site. Storing the vial in the refrigerator reduces this stinging sensation significantly.
• Metoclopramide (Reglan): A dopamine antagonist and prokinetic agent that stimulates upper GI motility.
  • Contraindication: Never administer metoclopramide if a gastrointestinal foreign body or obstruction is suspected, as the increased motility could lead to bowel perforation.
• Ondansetron (Zofran): A serotonin receptor antagonist used for severe, refractory vomiting (e.g., parvovirus or chemotherapy patients).

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Anesthetics and Analgesics

These drugs are used to eliminate pain (analgesia) and induce or maintain loss of sensation (anesthesia).

Analgesics

• Opioids: The gold standard for moderate to severe pain management.
  • Examples: Morphine, Hydromorphone, Fentanyl (often administered via transdermal patch or CRI), Buprenorphine (excellent sublingual/transmucosal absorption in cats), and Butorphanol (short-acting, mild analgesic/sedative, also used as a cough suppressant).
  • Side Effects: Respiratory depression, bradycardia, sedation, vomiting, constipation, and pupil changes (miosis in dogs, mydriasis in cats). Some patients may experience dysphoria (hallucinogenic distress).
• Gabapentin: An anticonvulsant that is widely used to treat neuropathic pain, chronic arthritic pain, and as a pre-visit sedative to reduce fear and anxiety (particularly in cats).

Anesthetics

• Induction Agents: Injectable drugs used to transition a patient from consciousness to unconsciousness so that an endotracheal tube can be placed.
  • Propofol: A short-acting, rapid-onset intravenous anesthetic. A common side effect is transient apnea (temporary cessation of breathing) if administered too rapidly. It must be given slowly to effect.
  • Alfaxalone: A neuroactive steroid molecule used for induction.
  • Ketamine: A dissociative anesthetic that leaves the eyes open (requiring lubricating ointment to prevent corneal ulcers) and keeps reflexes active. It is typically combined with a benzodiazepine (like diazepam or midazolam) to provide muscle relaxation.
• Inhalant Anesthetics: Gases mixed with oxygen to maintain surgical anesthesia.
  • Examples: Isoflurane, Sevoflurane.
  • Side Effects: Dose-dependent hypotension (low blood pressure), hypothermia (low body temperature), and respiratory depression.