Section 2 | Reading Livestock Medicines Labels

1. Calculating the Right Dose

There are four key questions you must answer to determine the right dose of a drug to give to an animal. Click each step below to read more and see an example:

• Step 1 | What is the animal’s weight?

  The amount of drug an animal needs is determined by the animal’s weight. To get the dosage right, it’s important to get an accurate weight for that animal. To estimate the weight of an animal, there are a few techniques that you could use:

  • Scales – The use of scales would give us the most accurate estimate of an animal’s weight
  • Use of validated weight tapes – There are a few tools in cattle and swine that relate animal dimensions (e.g. girth around chest) with weight
  • Estimate – based on experience or average weight for a given species, breed, and production stage
    • This is the least accurate method to estimate weight and should only be done if there are no viable alternatives available 

• Step 2 | What is your drug concentration?

  All bottles outline the concentration of active drug. This value is given on the label, and is normally in the form of milligrams per milliliter (mg/mL). Let’s look at a bottle of Liquamycin^® LA-200 as an example. On the label under the subtitle “Description” the company states:

  “Liquamycin^® LA-200 oxytetracycline injection is a unique formulation containing 200 mg oxytetracycline per mL…”

  This tells us that, for every milliliter (or “cc”) of Liquamycin^® LA-200, there is 200 milligrams of active product present. If we draw up 5 milliliters into a syringe, that means we have 1,000 milligrams (200 mg x 5 mL = 1,000 mg) of oxytetracycline.

• Step 3 | How much active drug do you need?

  This step involves figuring out how much active drug you need to treat an animal. There are 2 factors that affect how much active drug you need to treat an animal:

  • The appropriate dosage of active drug
    • The pharmaceutical company will have done studies to determine how much drug is needed to treat specific animals, and normally report this dose as the amount of active drug needed per unit bodyweight of the animal (normally mg/kg)
  • The weight of that animal (Step 1)

   

  Going back to our Liquamycin^® LA-200 example. The label states that the drug should be dosed at 20 mg oxytetracycline per kg bodyweight. So, if we are attempting to treat a 500 kg steer:

  • We need 20 mg of oxytetracycline for every 1 kg of weight
  • That means for an animal weighing 500 kg, we would need (500 kg x 20 mg/kg) mg of oxytetracycline = 10,000 mg of oxytetracycline

• Step 4 | How much do you need?

  This step requires you to calculate the total milliliters or cc’s needed to treat the animal. There are 2 things you need to know:

  • The total amount of active drug needed to treat the animal (Step 3)
  • The concentration of active drug in the product you are using (Step 2)

   

  Now all you need to do is divide the total amount of active drug needed to treat the animal by the concentration of active drug in the product you are using. For our Liquamycin^® LA-200 example, that would mean: 10,000 mg oxytetracycline / 200 mg oxytetracycline per mL = 50 millileters (or cc’s) of Liquamycin^® LA-200.