Fundamentals of veterinary pharmacology: classification of drugs and dosing methods

Proper application of veterinary drugs directly affects the survival rate of the herd and the economics of the livestock farm. Errors in dosage or choosing the wrong route of administration can completely neutralize the effect of the medication. It is important for a practicing veterinarian to calculate doses accurately and strictly observe the technology of drug preparation in farm conditions.

Classification of veterinary drugs and dosage calculation

The effect of any medication on the animal's body is regulated by the central nervous system and largely depends on the type of its higher nervous activity. All veterinary drugs are divided into 12 main pharmacological groups based on their composition and purpose. To obtain a therapeutic effect, it is important to choose the right route of administration and calculate the dosage of the active ingredient.

In veterinary practice, the following groups of drugs are used:

• antimicrobial and antiparasitic;
• mainly for local application;
• dietary and therapeutic-prophylactic;
• digestive aids;
• laxatives;
• expectorants;
• emetics;
• diuretics;
• acting directly on the nervous system;
• cardiac agents;
• influencing metabolic processes in the body;
• biogenic stimulants.

Depending on the clinical task, veterinary drugs are administered into the animal's body enterally or parenterally. The enteral route involves administration through the mouth or rectum, while the parenteral route involves subcutaneous, intramuscular, and intravenous injections. Drug formulations can be liquid, semi-solid, or solid.

• Dose calculation — per 1 kg of body weight
• Heating of infusions — 15 minutes
• Cooling of infusions — 40 minutes
• Heating of decoctions — 30 minutes
• Cold maceration — 4 hours
• Storage of decoctions and emulsions — 1–2 days

Technology for preparing liquid dosage forms

Liquid dosage forms require strict adherence to mixing technology, as errors can lead to precipitation or loss of substance activity. This group includes solutions, emulsions, mixtures, infusions, and decoctions. Each form has its own specificities regarding the dissolution of components and strict requirements for temperature conditions.

To prepare a standard solution, first pour in 1/3 to 1/2 of the solvent volume, then add the medicinal substance, mix it, and add the remaining liquid. Slowly soluble components that are heat-resistant are mixed in a hot medium. Finished solutions must be filtered, and injectable forms are sterilized by boiling or adding antiseptics.

Emulsions consist of fatty oils and an aqueous medium, which are combined using emulsifiers (starch, gelatin, lanolin). When preparing mixtures, first dissolve the easily soluble components, while the poorly soluble ones are pre-ground in a mortar. The resulting mixtures can be transparent, cloudy, or contain natural sediment.

Infusions and decoctions are obtained through aqueous extraction of active ingredients from plant raw materials. Infusions are prepared from the soft parts of plants (flowers, leaves, herbs), extracting components by hot or cold methods. For decoctions, dense parts are used — roots, bark, or wood, which require longer heating.

When preparing hot infusions, follow this procedure:

1. Grind the dry plant raw material by crushing, pounding, or cutting and place it in an infusion vessel.
2. Cover the raw material with clean cold water and keep it in a water or steam bath for 15 minutes.
3. Cool the mixture at room temperature for 40 minutes.
4. Strain the liquid through a strainer or canvas, filter through cotton wool, and carefully press the residue.

In the cold method, ground plant raw material is infused at room temperature for 4 hours without heating. To prepare decoctions, ground bark or roots are covered with cold water in an infusion vessel and heated in a water bath for 30 minutes. The resulting liquid is strained while hot and cooled immediately before use.

Preparation and application of semi-solid dosage forms

Semi-solid dosage forms are used in livestock farming to treat skin, mucous membranes, and wounds. These include ointments, pastes, liniments, and mashes. A proper choice of base and adherence to mixing technology allow for a stable drug that will not separate during storage and will provide maximum therapeutic effect.

Ointments consist of an active substance and a base (lard, lanolin, petroleum jelly, paraffin, or beeswax). They melt at a temperature slightly above the animal's body temperature, making them easy to spread on the skin. When preparing ointments on one's own at a veterinary station, it is important to follow the exact sequence of actions.

1. Grind the dry or liquid components prescribed in the smallest quantities in a mortar.
2. Gradually add the other active ingredients, mixing them with a small amount of the base.
3. Add the remaining base in portions while stirring constantly.
4. Grind the mass until uniform and transfer it with a spatula into a glass or porcelain jar.

Pastes contain a higher concentration of solids, which makes them thick and allows them to remain on the affected area for longer. Liniments are liquid ointments based on vegetable oil (sunflower, linseed) or liquid soap, which are easily distributed over the fur. For oral administration, mashes are prepared — dough-like mixtures of active ingredients with binding components.

Solid dosage forms and their dosing features

Solid dosage forms are convenient for precise individual dosing and long-term storage on the farm. In veterinary medicine, powders, tablets, and pills are the most common. They can be used for both mass therapeutic and prophylactic treatments (by mixing the drugs into feed) and individual therapy.

Powders can be simple or complex, dosed or non-dosed. To prepare them, raw materials are ground in mortars or mills. Plant materials must be prepared before grinding to prevent spoilage and preserve the activity of the active ingredients.

Tablets weighing from 0.3 to 1 g are manufactured at a factory by adding starch or talc. The presence of score lines allows them to be divided into parts for precise dose adjustment for a specific animal. Pills are molded into the shape of balls for slow dissolution in the stomach and intestines, which ensures a prolonged effect of the drug.

When hand-molding pills, it is important to select the auxiliary components correctly:

• Liquid and semi-liquid ingredients (water, syrups, glycerin) are added to dry medicinal substances.
• Powdered fillers (crushed licorice, marshmallow, or dandelion roots, gum arabic, or white clay) are mixed into liquid preparations.

• Proportion of powders in pastes — 25 % or more
• Drying temperature for plant materials — 40–50 °С
• Mass of veterinary tablets — 0.3–1 g
• Dividing the tablet along score lines — into 2–4 parts

Manufacturing rules and methods of administering boluses and pills

When manufacturing solid dosage forms directly on the farm, it is important to strictly observe the mixing technology. To prepare pills, all dry components are weighed, thoroughly ground, and sifted. Liquid ingredients are added gradually: first, they are poured into the mortar onto a layer of inert powder, mixed until uniform, and only after that combined with the rest of the dry base.

1. Weigh, grind in a mortar, and sift the dry components of the drug.
2. Pour the liquid substances onto the layer of inert powder in the mortar and mix until uniform.
3. Add the remaining dry base and knead into a dense, non-crumbling mass.
4. Transfer the mass to the board of a pill machine and form a cylinder using a glass plate.
5. Cut the cylinder with a cutter into 25–30 equal doses, give the pieces a rounded shape, and sprinkle with inert powder.

For large animals, a more convenient form is the bolus — a soft pill weighing more than 0.5 g. It is prepared from the ground active ingredient and a forming base, such as marshmallow root, flour, or molasses. The mixed dough-like mass is processed in a mortar, divided into portions, and rolled into balls by hand.

• Bolus mass for large animals — 30 g
• Bolus mass for dogs and poultry — 3–5 g
• Shelf life of fresh boluses — 2–3 days

Do not prepare large quantities of boluses in advance. Due to the lack of preservatives, they dry out quickly and lose their necessary soft consistency within 2–3 days.

The method of administering solid and liquid preparations is chosen based on the type of animal. Powders are easiest to give with feed: for cattle, they are mixed into concentrates; for pigs and cats, into milk; for dogs, into minced meat. Boluses, tablets, and pills are administered to large livestock and horses using a bolus gun or a long syringe without a needle, pushing the drug onto the root of the tongue for rapid swallowing. Liquid medicines are poured for large animals from a special rubber, polyethylene, or thick-walled glass bottle.

Liquid dosage forms must be administered with extreme caution. Introduce the solution slowly and in portions to prevent the preparation from entering the trachea and causing aspiration pneumonia.

Calculation of therapeutic doses and alternative routes of administration

The therapeutic dose of the same active ingredient is not constant. It depends directly on the species of the livestock animal, its body weight, sex, age, as well as the route of administration and the general condition of the organism. The preparation is always calculated per 1 kg of body weight, taking into account interspecies ratios.

Species of livestock animal	Body weight, kg	Dose ratio (in parts)
Horse	400	1
Cattle	300–350	1–1,5

Sensitivity to preparations varies significantly depending on the physiological state. Females react to medications more strongly than males, especially during pregnancy. Young and old animals are also more sensitive to active ingredients than individuals of middle age. In addition, sick animals are more susceptible than healthy ones, and a change in dosage can completely alter the nature of the preparation's effect. For example, rhubarb extract in small doses improves digestion, in medium doses slows down peristalsis, and in large doses acts as a laxative.

For external therapy, the affected skin areas are prepared beforehand: hair is clipped, the skin is washed with warm water and dried. Powders are applied using powder blowers or gauze bags, while ointments, pastes, and liniments are applied with a spatula, brush, or swab. Liquid forms are used for rubbing, compresses, and poultices, while mucous membranes are washed with syringes or through a rubber tube with a funnel.

For the administration of liquids through the rectum (rectally), an Esmarch irrigator with a rubber tube is used for large animals, or a syringe for small ones.

For parenteral administration, strictly observe the rules for selecting solvents for each injection route:

• Subcutaneously: only aqueous and oil solutions are allowed to be administered.
• Intramuscularly: in addition to aqueous and oil solutions, suspensions can be used.
• Intravenously: exclusively sterile aqueous solutions are administered.
• Intraperitoneally: primarily sterile aqueous-saline solutions are used.

For a rapid effect on the respiratory system, the inhalation method is used. Vaporous substances, gases, and aerosols are administered using special inhalers and aerosol generators, or the animals are kept in equipped hermetic chambers with air saturated with the medication.

Catheterization and preparations affecting the nervous and cardiovascular systems

For the treatment of the mammary gland, medications are introduced through the teat canal using a milk catheter. Into the uterus and vagina, solutions are delivered using a syringe or an Esmarch irrigator. Catheterization of the urinary tract is performed with special veterinary catheters, while for small animals, medical analogs are used.

Veterinary preparations are grouped by the nature of their main effect on the organism. This allows for quick orientation in prescriptions and precise selection of therapy for specific symptoms. Medicinal agents are divided into the following main groups:

• substances acting on the central nervous system;
• substances acting primarily in the area of nerve endings;
• emollient and mucilaginous agents;
• diuretics;
• cardiovascular agents;
• substances with a primary influence on tissue metabolism processes;
• biogenic stimulants;
• antimicrobial and antiparasitic agents.

Preparations affecting the central nervous system regulate higher nervous activity. Depending on the clinical picture, they are used to stimulate or calm the animals. To tone the cerebral cortex, caffeine preparations (caffeine-sodium benzoate) and camphor (cordiamine, camphor oil) are used. Inhibition of nervous processes is caused by bromine preparations — potassium, sodium, and ammonium bromides.

For anesthesia, inhalation (ether, chloroform) and non-inhalation agents (chloral hydrate, veronal) are used. This same group includes emetics, ruminatories, and expectorants: apomorphine, white hellebore rhizome extract, and ipecacuanha root. Analgesic, anti-rheumatic, and antipyretic effects are provided by analgin, antifebrin, and acetylsalicylic acid.

Substances acting in the area of nerve endings are divided into irritants, astringents, emollients, and mucilaginous agents. Irritants include ammonia, mustard, and turpentine, which are applied externally in a mixture with oil or soap solutions. Turpentine is also prescribed internally in the form of emulsions as an anti-fermentative and ruminatory agent, and for lung diseases, it is used for inhalations together with tar and ichthyol.

Internally, animals are often given infusions of medicinal herbs containing essential oils. Chamomile flower infusion works as an anti-inflammatory and anti-fermentative agent. Valerian root and rhizome infusion relieves intestinal spasms, calms nervous excitement, and supports heart function during its decline.

To protect mucous membranes, enveloping decoctions of flaxseed, starch, or marshmallow root are used. Externally, the skin is protected with petrolatum, paraffin, as well as plant and livestock animal fats. For gastritis and gastroenteritis with diarrhea, astringents are prescribed, for example, tannin from oak bark or burnet rhizome, which tighten tissues by precipitating proteins.

Arecoline and pilocarpine are administered subcutaneously with great caution, as they strongly affect the autonomic nervous system. For a milder laxative effect, castor oil or aloe (bitter aloe) are used, which moderately irritate intestinal receptors.

Cardiovascular agents based on foxglove, lily of the valley, and pheasant's eye contain cardiac glycosides. These substances increase the force of myocardial contractions while simultaneously slowing the heart rhythm. By lengthening the relaxation period (diastole), the heart receives the necessary rest, which normalizes blood circulation.

Diuretics and metabolic correction agents

Diuretics increase fluid excretion and accelerate plasma filtration in the renal glomeruli. These include diuretin, which acts through the central nervous system, and mercurzan, which affects tissue metabolism. To disinfect the kidneys and urinary tract, preparations with disinfectant action are used — urotropin and bearberry leaves.

Preparations for tissue metabolism include hormones, enzymes, vitamins, acids, alkalis, and metal compounds. In the case of a deficient diet, element shortages are compensated with vitamin-rich feed and concentrates. General administration of acids accelerates the breakdown of proteins, carbohydrates, and partially fats, while alkalis are often used as neutralizers in cases of acid poisoning.

Phosphorus and calcium supplements are administered strictly according to calculations based on their ratio in the feed and the current blood parameters of the livestock animals. This helps to avoid severe mineral metabolism disorders.

Trace elements — compounds of iodine, cobalt, copper, manganese, zinc, and selenium — are introduced into the soil as part of micronutrient fertilizers or added to feed and water. The effect of metal salts largely depends on the specificity of the tissues and the current physiological state of the livestock animal. Systematic application of such supplements is critically important in regions with mineral-deficient soil.

Biogenic stimulants are biologically active substances with complex action that accelerate vital processes in the body, increasing the resistance and productivity of livestock animals. This group includes protein preparations, serums, cytotoxins, hormones, and tissue preparations. Their application requires a systematic approach to livestock management.

Remember that biogenic stimulants show maximum effectiveness only when accompanied by proper feeding and favorable conditions for the livestock animals.

Antimicrobial and antiparasitic agents are used to combat pathogens. Their action directly depends on the condition of the livestock animal and the selective properties of the preparation itself. In veterinary medicine, three main groups of such substances are actively used:

• Antibiotics (penicillin, streptomycin, levomycetin, erythromycin, tetracycline) — selectively suppress pathogenic microflora and are administered intramuscularly, intravenously, or with feed;
• Sulfonamides (white streptocide, norsulfazole, disulfan, phthalazol) — derivatives of sulfanilic acid, which are prescribed orally, often in combination with antibiotics;
• Nitrofuran derivatives (furacilin, furazolidone, furadonin, furazol) — provide both bacteriostatic and bacteriolytic effects.

In addition to these groups, phenols, plant-based insecticides (pyrethrins), formalin (40% formaldehyde), alkalis, and chlorine, sulfur, and iodine preparations are used as antimicrobial and antiparasitic agents. Dyes (flavacridine, ethacridine lactate/rivanol) and anthelmintics are also used. Any preparation requires strict adherence to indications and dosages.

Rules for storage and inventory of veterinary preparations

Treatment efficacy depends not only on dosage accuracy but also on drug storage conditions. Violation of storage rules leads to the loss of therapeutic properties and the degradation of active ingredients. Organize storage on the farm using the following algorithm:

1. Ensure low temperatures for preparations that spoil quickly under the influence of heat.
2. Place volatile substances in hermetically sealed containers.
3. Protect compounds that decompose in light by distributing them into dark glass containers or dark cabinets.
4. Place iodine and bleaching powder in a separate dark place in tightly sealed containers.
5. Protect dry hygroscopic substances from moisture by placing them in cabinets in a dry room.
6. Equip isolated cellar areas for storing flammable substances and biological preparations (vaccines, serums).

Keep disinfectants in places completely inaccessible to unauthorized persons and livestock animals. All incoming and outgoing medications must be recorded in special logbooks.