Responsible Use of Veterinary Drugs

Everyone who uses drugs on animals has a responsibility to ensure that they are used properly and that their use does not result in drug residues in the human food chain. This factsheet describes how to administer drugs in a responsible manner and also indicates some of the more frequent causes of drug residues.

Health Canada's Veterinary Drug Directorate has established tolerances for a number of chemical and biological residues in meat and other food products. Monitoring to ensure these limits are not exceeded is carried out on a random basis except when violative levels have been detected in the past or there are other reasons to suspect there may be a concern.

The leading cause of residues in meat and milk is the failure to observe withdrawal times. Other factors responsible for residues include exceeding the recommended dose, improper mixing of feed, and mistaken administration due to not reviewing the label, lack of animal identification and record of treatment.

Also, common over-the counter injectable drugs are more frequently involved with residues than prescription drugs. The person who administers drugs and makes decisions on the sale of the animals is ultimately responsible for the majority of drug violations.

Label Directions

Read the label before using any drug or vaccine. All drugs should be used according to label directions. The following are some of the more important points to observe.

Dosage
Dosage is usually determined by the body weight of the animals undergoing treatment, so weigh or estimate an animal's weight as accurately as possible and calculate the dosage accordingly. Exceeding the dose printed on the label constitutes extra label use (see below) and is not allowed without veterinary oversight. If a drug is not working at the dosage indicated on the label, then another appropriate drug should be used or the diagnosis re-evaluated. Consultation with a veterinarian should also be considered.

Frequency of administration
Treatment frequency or interval can range with different treatment products. Some may be administered more than once a day while others may be administered once a day or even once every few days. The label will indicate how often a drug should be given. The duration of treatment varies with the drug being used, the disease or condition being treated, and the response of the animal.

Species
Some drugs are not cleared for use in all species. Ensure that the drug you are using has been approved for the species and class of animals you intend to treat. Otherwise, this is another situation that would be considered an extra label usage.

Route of administration
The route of administration can greatly affect how quickly the drug gets to the site of infection and how quickly it is cleared from the animal’s body. Administer drugs according to label requirements for maximum effectiveness of the drug, safety of the animal, and ultimately the safety of food products derived from that animal.

Withdrawal time
All antibacterial preparations and many other drugs have a withdrawal time. This is the amount of time that must elapse after the last dose of the drug was administered to the animal before it can be slaughtered or any of its products used for food. Withdrawal times vary considerably depending upon the dose and route of administration, individual drug and the formulation of the drug that is used.

Expiration date
All injectable antibiotics and vaccines, many oral products and some other drugs have an expiration date stamped on the label and on the box in which the drug is packed. Exp 31 12 18 or Exp 12 18 means that the product should not be used after December 31st, 2018. Antibiotics degrade over time and can form harmful breakdown products while vaccines lose their potency. Some drugs expire more quickly than others. The expiry date is determined by the manufacturer and indicates that the product should be perfectly safe and effective until the expiry date provided it has been stored and handled properly.

Storage requirements
Some drugs may be stored at room temperature while others require a cooler temperature which is indicated on the label. Drugs that require storage at 2 to 6°C should be kept in a refrigerator. Others require storage in a cool place between 8 and 15°C. All drugs should be protected from freezing and excessive heat. Medicated feeds should be stored separately from non-medicated feeds.

Drug Administration

The route by which a drug is administered to an animal depends on the chemical and physical properties of the drug, how quickly a response to the drug is required and the site in the body where one wishes the drug to act. Some drugs are restricted to certain routes of administration and may cause irritation or adverse reactions if given via other routes. It is vitally important that drugs be administered by the proper route.

Oral administration
Drugs can be given by mouth in several ways. They may be mixed with feed or, if soluble, added to the drinking water. Pills or boluses may be given by hand or with a balling gun. Insoluble drugs or liquids in small quantities may be administered as a drench or through a stomach tube. Drugs given by mouth must survive passage through the stomach and into the small intestine before they are absorbed. Their action is, therefore, relatively slow.

Injections
This is probably the most frequently used method of administering drugs, particularly when treating single or small groups of animals.

Subcutaneous injection is placed under the skin and is a common method used for injectable antibiotics. Although the speed of action is not as fast as when given intravenously or intramuscularly, the rate depends upon the base in which the drug is suspended. An aqueous suspension is absorbed more quickly than an oily one.

The intramuscular route is given into the muscle and is the method used for some antibiotics. Care should be taken to administer the drug into the neck section of the animal where lower value cuts of meat are derived from.

Intravenous injections are given right into a vein, often the jugular. The solution injected must be sterile and the response obtained is the most rapid of any route of injection. It is also the route of choice for irritating drugs since they are diluted by blood during administration.

Sterile and non-irritating drugs can also be injected by the intraperitoneal route, i.e., into the abdomen. Absorption is fairly rapid because of the large surface area exposed to the drug but is not quite as rapid as intravenous. This procedure requires knowledge of the anatomy of the animal and should not be undertaken without instruction from a veterinarian.

lntramammary injections are used to treat mastitis in dairy cattle, typically with an antibiotic being injected via the teat. .

Local treatment
Drugs may also be administered locally to many parts of the body such as the eyes, ears, nose and uterus. Ointments may be applied to the skin and the active ingredient will be absorbed and distributed throughout the body if a watermiscible base is used. If not, the drug will remain locally. Growth implants are usually inserted under the skin of the ear and the drug is released from the implant slowly.

Mixing of drugs
Drugs should never be mixed in the same syringe or administered to an animal at the same time unless they are compatible. If there is any doubt as to the compatibility of two drugs, the advice of a veterinarian should be sought. Not only are some drugs incompatible, they work against each other thus nullifying the effect of both. When administering drugs via feed, the directions contained in the Medicating Ingredient Brochures (published by Agriculture Canada) must be followed. Drug combinations not listed in the accepted compatibilities for each drug should not be used.

"Recycling" of drugs
Many drugs are excreted in the feces and urine. Precautions should be taken to avoid recycling of these drugs through consumption of contaminated bedding or contact with manure or urine.

Extra label use
Extra label drug use is a term used to describe administration of a drug at a level higher than that indicated on the label or use in a species for which that drug has not been cleared. Administration by a route other than that recommended on the label or at a stage in the production cycle of an animal for which the drug has not been approved also constitutes extra label use. An example of the latter would be the use of a dry cow mastitis preparation in lactating cows. Dry cow preparations are usually in a slow release base, which will result in milk residues for a long period of time.

The primary problem with extra label use is that withdrawal times are not established. Resources are available for your veterinarian to help establish a withdrawal time.

Disposal of Drugs

Outdated or unwanted drugs should not be thrown into the garbage. They should be incinerated if such facilities are available or, alternatively, contact should be made with your local veterinary clinic or pharmacy to advise of procedures in your community.