Merck Manual

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Pet Owner Version

How Drugs are Given in Animals


Philip T. Reeves

, BVSc (Hons), PhD, FANZCVS, Veterinary Medicines and Nanotechnology, Australian Pesticides and Veterinary Medicines Authority;

Camille Roesch

, PhD, Australian Pesticides and Veterinary Medicines Authority, Symonston, Australia;

Michelle Nic Raghnaill

, PhD, National Academy of Science, Canberra, Australia

Reviewed/Revised Jul 2011 | Modified Nov 2022

A wide range of dosage formulations and delivery systems has been developed to provide for the care and welfare of animals. Using the correct dosage is very important in terms of effectiveness and safety. Drug treatment and delivery strategies can be complicated because of the variety of species and breeds treated, the wide range in body sizes, different animal rearing practices, seasonal variations, and the level of convenience, among other factors.

Drugs Given by Mouth

Oral dosage forms (given by mouth) include liquids (solutions, suspensions, and emulsions), semi-solids (pastes), and solids (tablets, capsules, powders, granules, premixes, and medicated blocks).

A solution is a mixture of 2 or more components that mix well and form a single phase that is consistent down to the molecular level (such as sugar water). Solutions are absorbed quickly and generally cause little irritation of the lining of the stomach and intestine. However, the taste of some drugs is more unpleasant when in solution. Oral solutions provide a convenient means of drug administration to newborn and young animals.

A suspension is a coarse dispersion of insoluble drug particles in a liquid (for example, flour mixed in water). Suspensions are useful for administering insoluble or poorly soluble drugs or in situations when the presence of a finely divided form of the material in the stomach and intestinal tract is required. The taste of most drugs is less noticeable in suspension than in solution because the drug is less soluble in suspension. Suspensions must typically be shaken vigorously just prior to administering.

An emulsion consists of 2 non-mixable liquids, one of which is dispersed throughout the other in the form of fine droplets (such as oil and vinegar salad dressing). Emulsions for oral administration are usually oil (the active ingredient) in water. They facilitate the administration of oily substances such as castor oil or liquid paraffin in a more palatable form.

A paste is a 2-component semi-solid in which a drug is dispersed as a powder in a liquid or fatty base. It is critical that pastes have a pleasant taste or are tasteless. Pastes are a popular dosage form for treating cats and horses, and can be easily and safely administered by owners.

A tablet consists of one or more active ingredients mixed with fillers. It may be a conventional tablet that is swallowed whole or a chewable tablet. Conventional and chewable tablets are the most common forms used to administer drugs to dogs and cats. Tablets can be more physically and chemically stable than liquid forms. The main disadvantages of tablets are the low absorption rate of poorly water-soluble drugs or simply poorly absorbed drugs, and the local irritation of the stomach or digestive tract lining that some drugs may cause.

A capsule is usually made from gelatin and filled with an active ingredient and fillers. Two common capsule types are available: hard gelatin capsules for solid-fill formulas, and soft gelatin capsules for liquid-fill or semi-solid-fill formulas. Capsules have no taste and are therefore good for drugs that are otherwise hard to give because of their bad flavor.

A powder is a formulation in which a drug powder is mixed with other powdered fillers to produce a final product. Most powders are added to food. Powders have better chemical stability than liquids and dissolve faster than tablets or capsules. Unpleasant tastes can be a factor with powders and are a particular concern when used in food because the animal may not eat all of it. In addition, sick animals often eat less and may not eat enough of the powdered drug for it to be effective.

A granule consists of powder particles that have been formed into larger pieces. Granulation is used when combining more than one form of medication. Granulation is especially effective for combining particles that are of different sizes because it helps prevent the separation or settling of the different particle sizes during storage or dose administration. Imagine granola clusters—if you just have granola mix, the smaller pieces fall to the bottom and are not eaten as often, but if you form it into clusters (large granules), you get every type of ingredient in each bite.

Drugs Given as Injections

A drug that is given parenterally—that is, by injection—does not go through the gastrointestinal system. These drugs may be formulated in several different ways for use in animals, including solutions, suspensions, emulsions ( see How Drugs are Given in Animals How Drugs are Given in Animals A wide range of dosage formulations and delivery systems has been developed to provide for the care and welfare of animals. Using the correct dosage is very important in terms of effectiveness... read more ), and as a dry powder that is mixed with a liquid to become a solution or a suspension immediately prior to injection. Dry powders are used for those drugs that are unstable in liquid form.

Drugs Applied to the Skin or Mucous Membranes

The dosage forms applied to the skin or mucous membranes that are available for treating animals include solids (dusting powders), semi-solids (creams, ointments, and pastes), and liquids (solutions, suspension concentrates, and emulsifiable concentrates). These are known as topical drugs. Of special interest are transdermal delivery systems that work by carrying medications across the skin barrier to the bloodstream. Examples of these are transdermal gels and patches that are used in pets. There are also dosage forms that are unique to veterinary medicine, such as spot-on or pour-on formulations developed for the control of parasites.

A dusting powder is a fine-textured insoluble powder containing ingredients such as talc, zinc oxide, or starch in addition to the drug. Some feel gritty, and some have a smooth texture. Some dusting powders absorb moisture, which discourages bacterial growth. Others are used for their lubricant properties. The use of dusting powders is good for skin folds and not good for use on wet surfaces, as caking and clumping is likely to result.

A cream is a semi-solid emulsion formulated for application to the skin or mucous membranes. Cream emulsions are most commonly oil-in-water but may be water-in-oil. The oil-in-water creams easily rub into the skin (commonly called vanishing creams), and are easily removed by licking and washing. Water-in-oil emulsions are skin-softening and cleansing. Water-in-oil creams are also less greasy and spread more readily than ointments.

An ointment is a greasy, semi-solid preparation that contains dissolved or dispersed drugs. A range of ointment bases is used. Ointments are often effective at soothing because they block the skin from irritation. Ointments are useful for chronic, dry skin conditions and are not good for oozing or weeping areas of the skin.

A paste for skin use is a stiff preparation containing a high proportion of finely powdered solids such as starch, zinc oxide, calcium carbonate, and talc in addition to the drug(s). Pastes are less greasy than ointments. Pastes do not seal wounds.

Solutions are liquid formulations ( see Drugs Given by Mouth Drugs Given by Mouth ). Topical solutions include eye drops, ear drops, and lotions.

A transdermal delivery gel consists of a gel that delivers the active drug through the skin to the bloodstream. Not all drugs are suitable for this type of transdermal application, however. Transdermal gels have been used to treat several diseases in dogs and cats, including undesirable behavior, cardiac disease, and hyperthyroidism. The dose is applied to the inner surface of the ear, making it easy to administer, especially in cats.

A transdermal delivery patch typically consists of a drug incorporated into a patch that is applied to the skin. The drug is absorbed across the skin over a long period of time. One type of pain reliever, which produces reactions like the body’s own natural pain relievers, is delivered by transdermal patch in dogs, cats, and horses.

A spot-on formulation is a solution of active ingredient(s), which also typically contains a co-solvent and a spreading agent to ensure that the product is distributed to the entire body.

Insecticidal collars are plasticized polymer resins that contain an active ingredient. Collars for the control of ticks and fleas on dogs and cats release the active ingredient as a vapor, a dust, or a liquid, depending on the chemical. The animal’s activity is an important factor in how well the insecticide moves from the collar to the animal.

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