Another factor that can affect the horse's responsiveness to a drug is the condition of the digestive system. When administering an oral drug, the extent of absorption will differ depending on conditions in the gut.

 "The pH (a measure of acidity) of the stomach acid varies within a normal range in a horse," explains Schaumberg."Diet, hormonal influences on stomach mucosa, stress, and many other factors affect the stomach pH. A one-point change in pH can dramatically influence the stomach's absorption of a drug."

  An example of this might be if the horse suffers from diarrhea. The effectiveness of the drug will generally be decreased due to the rapid rate of passage through the gut. On the flip side, constipation might enhance the absorption of the drug due to slowed movement through the intestinal tract.

   "Drugs are also excreted through the kidneys, as well as through sweat or exhalation, and can be excreted whole or broken down as metabolites," adds Schaumberg. "For instance, urinary excretion is also affected by pH. Acidic urine increases the excretion of basic drugs, while a more alkaline urine readily excretes drugs with a low pH."Wilson, adds, "Many drugs are metabolized by the liver and eliminated either in the bile or re-circulated through the kidneys to be eliminated in urine."Other factors that must be taken into account when selecting a drug and deciding on the proper dosage for that particular horse include determining the horse's  temperament, general health, body lean to fat ratio, and whether the drug is safe for a pregnant mare (if that's the intended use).

     "The bottom line is, you should know the range of dosage in accordance with the horse's weight and age and adjust up and down according to the factors that you are aware of," advises Schaumberg. "This is why doing a blood work-up to establish cholesterol, triglyceride, or albumin (proteins that are soluble in water) levels allows the veterinarian to make a more informed decision. "Wilson also stresses the importance in the evaluation of both the kidney and liver functions.  

      While many drugs can be safely used together, it can be potentially dangerous to the horse to administer more than one drug. For this reason, when administering any drug, it's very important to understand how it will affect the horse in the presence of an additional drug. "There are literally hundreds of drug interactions," says Schaumberg. "An antibiotic like tetracycline is bacteriostatic (slows growth) will interfere with an antibiotic like penicillin that is bactericidal (kills only growing bacteria). Drugs, like Lasix (a diuretic also known as Salix), will cause some drugs that are excreted by the kidneys to be excreted faster."

     Have diligent records of drug usage, and keep these records handy so they can be referenced prior to administering any medication. If there is a question of drug compatibility, you should consult your veterinarian before administering a second drug. This includes supplements and dewormers. For example, any tranquilizer in the promazine group cannot be used safely if a horse has recently been dewormed with a piperazine, phenothiazine or organophosphate product, as these will significantly increase the likelihood of toxicity.

     With only a few exceptions, drug dosage is presented as a weight unit of the drug for each weight unit of the horse. Keep in mind formulations of like drugs are not always equal. They can vary in the concentration of active and inactive ingredients depending on the manufacturer of the drug. In particular, concentrations vary in injectables, such as the vitamin B complexes and various antibacterial agents.

     When medicating a horse, the owner must always fulfill his/her responsibilities. This means you must attempt to understand the function of the chemicals you plan to use, how each drug works, its intended use, and what it will or will not do. 

     Medications should never be used for a purpose that differs from what they are designated to do unless prescribed by a veterinarian. At no time should a drug be used in an indiscriminate manner, such as on a horse other than the horse that the drug was the originally prescribed for. Neither should a drug specifically produced for horse be used on other animals or humans.

     If properly prescribed and administered, drugs establish a valued defense in the treatment of many illnesses and injuries that affect the horse.

Safe Management of Drugs

• Consult your veterinarian if you are unsure of a particular health concern and the appropriate treatment for your horse.

• Drugs, whatever the type, should be stored far out of reach of children, away from exposure to direct sunlight, and within the appropriate temperature range suggested by the manufacturer.

• Always note the expiration date on drug labels and avoid using any product that has expired or appears to have discolored. Even if the expiration date is okay, if it looks odd to you in any way don't use it!

• Always dispense drugs according to the manufacturer's or veterinarian's directions, whether topically, orally, or by injection (i.e., intramuscular, subcutaneous, etc.). Some drugs can only be given intramuscularly, such as procaine penicillin and most vaccines. Others, like the injectable form of phenylbutazone (Bute), are very irritating to the muscles and should only be given through an intravenous injection.

• Know how to administer drugs properly. Get help or advice from your veterinarian if you are unsure of the proper sites and methods used when administering an injectable medication.

• Stick to proper hygiene when giving an injectable medication--avoid contamination of the bottle. Passing dirty needles through a dirty top will introduce bacteria that can cause serious infections. Appropriate cleansing of the injection site is also important to help prevent contamination of the underlying tissues.

• Never mix products unless specifically recommended by the manufacturer.

• Do not save partially used vaccine vials for later use.

• When using a multiple-dose vial, never enter the container with an unsterile needle.

• Properly disinfect all re-usable equipment, specifically syringes, needles, etc. Strong chemical disinfectants that might be inadvertently left remaining in a syringe can destroy both live and modified live biologicals. For this reason, it's best to boil equipment in distilled water for 20 minutes rather than sterilizing in alcohol or other disinfectants.

• Dispose of used containers, vials, needles, and syringes properly. Do not leave them where they will become a hazard to man or animal. (Beware of needles dropping in bedding!)

• Along with the product's caution statement for appropriate handling, read the directions for the drug's proper use and method of administration, then follow them exactly.

• Take the time to learn about the possible side effects of each drug you intend to use and what emergency steps you must take should your horse experience a reaction to the drug.