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The amount of active ingredient in a drug is commonly referred to as its strength. For example, an extra strength Tylenol capsule contains 500mg of active ingredient. Therefore, 500mg is its strength.
For liquid drugs, both oral and injectable, concentration is an important factor in dosing. Concentration refers to the amount of active drug (strength) in a given volume of vehicle, usually a liquid, but could be a paste, etc. For example, the concentration of Panacur suspension is 100mg/ml. This means that in every 1ml of liquid there is 100 mg of active drug.
Concentration = Strength / Volume
Concentration = 100mg / 1ml
Another factor that is often required when calculating doses is the animal's weight. The weight of the animal affects how the drug is absorbed, distributed into the tissues and utilized by the animal's body. Generally, the more the animal weighs, the more drug is required for the desired effect. It is important to use an accurate weight in your calculations, otherwise, you may be giving too little or too much medication (see article on importance of weighing).
Recommended Doses
Because weight is such an important factor, most drug dosages you will see in reference tables and prescribed by your vet will be written as giving so much drug (strength) per pound (lb) or kilogram (kg) of body weight. For example, you will see the prescribed dose of Ivomec often expressed as 1 ml / 70 lbs. This simple means that a 70 lb alpaca needs 1ml Ivomec. But what if your alpaca doesn't weigh 70 lbs.? This is where we need to use a simple formula to determine the amount to give.
Suppose your alpaca weighs 165 lbs. Here is the formula to use:
weight of alpaca x prescribed dose = amount to give
165 lbs. x

1ml

=

165 lbs. ml

= 2.4 ml

70 lbs.

70 lbs.

(Remember when multiplying fractions you multiply the numerator (top number)
and divide by then denominator. The same units above and below the line cancel out)
Because the Metric system of measurement is commonly used in the sciences, often you may come across dose recommendations that are based on kilograms (kg) not pounds (lbs.). You could set your scale to weigh in kilograms so you don't have to convert units of measure, but most of us are more familiar with pounds. So, we need to convert the weight of our alpaca to kilograms and then use the formula above.
For example, you are told to dose your alpaca at 1ml / 32 Kg. Your alpaca still weighs 165 lbs. Here is the conversion formula:
Weight in Kg =

Weight in lbs.

2.2

or


75 kg = < /FONT >

165 lbs.

2.2

Now that we know our alpaca weighs 75kg, plug the numbers into the same formula we used above:
weight of alpaca x prescribed dose = amount to give
75 Kg x

1ml

=

75 Kg ml

= 2.3 ml

32 Kg

32 Kg

The above formulas work well when you are instructed to give a certain volume (ml) of medication based on weight of your alpaca. But suppose your vet already knows the weight of your alpaca and she tells you to give so many milligrams (mg) of a liquid medication. In this case, you need to use a slightly different formula.
For example, you are told to give your cria 150mg of SMZ orally. You look on the bottle and see the drug concentration listed as 200 mg/5ml. Remember that:
Concentration = Strength / Volume
So, 200mg = the strength and 5ml = the volume. With that in mind, here is the formula:
prescribed dose
strength

x volume =

= amount to give


or




150 mg
200 mg

x 5 ml =

750 mg ml
200 mg

= 3.8 ml






Remember: Before doing the calculation, convert units of measurement to one system.
Common Conversions:
1 Liter = 1000 ml
1 ml = 1cc
1 Gram = 1000 mg
1 kg = 2.2 pounds