Medical Dosage Calculations & Formulas

Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will delineate dose vs. dosage and teach you how to calculate a drug dose for solid dosage forms and liquid dosage forms (where you need to know the drug's concentration).

Dose Vs. Dosage

When a patient is set to receive a medication, we need to know what the dose of that drug is going to be. The drug dose, sometimes called the absolute dose, is the total quantity of a drug that is to be given to a patient. There's also something known as the relative dose, a dose relative to the patient's body weight.

  • 5 mg is a dose. 5 mg/kg is a relative dose.

And if that wasn't confusing enough, another term that you should be aware of is dosage, the frequency at which a dose is administered.

  • 500 mg is a dose. 5 mg/kg is a relative dose. 500 mg every 12 hours is the dosage.

This lesson will teach you how to calculate solid and liquid drug doses when you have a patient's weight, the relative dose referenced from an appropriate source and, if need be, the concentration of a liquid dosage form.

Calculating Doses For Solids

Usually, when you're going to be calculating the drug dose for a medication that comes in a solid dosage form, like a tablet, capsule, or pill, you'll be calculating out the dose in mg (milligrams).

The very first thing you'll need to know is the patient's weight. This weight may be in kilograms (kg), not in pounds (lb), so always pay attention to your units! You should always remember that there are 2.2 pounds in each kilogram (2.2 lb/kg).

Once you have the patient's weight in kilograms, you need to calculate the total dose using the relative dose. The relative dose is often given in units of 'mg/kg'.

So, let's practice, shall we? Let's say the patient weighs exactly 10 kg. The relative dose is 15 mg/kg. What do we do? We always multiply the two numbers to get the dose. When you multiply 'kg' by 'mg/kg', the 'kg' cancel out and you're left with 'mg'. So:

  • 10 kg * 15 mg/kg = 150 mg.

150 mg is our dose. If the patient is to be given this amount once per day, the dosage is 150 mg/day.

Calculating Doses For Liquids

Now, that was pretty easy, wasn't it? But when we calculate the dose for a drug in a liquid form, things get a bit trickier. That's because we have to take into account the drug's concentration, the amount of dissolved substance in a set volume of solvent.

If you dissolve one teaspoon of sugar in one cup of water (the solvent), then the concentration will be X. But if you dissolve two teaspoons of sugar in a cup of water, the concentration will be 2X.

Concentrations are very important in medicine because you can easily mess up the dose you are calculating if you use the wrong concentration! That could either mean the amount you'll eventually give won't have the therapeutic effect you want, or, on the flipside, it can kill your patient.

When we tell someone the dose of a medication that is found in liquid form, we do not tell them it is X mg, like we do for solid dosage forms.

For liquid dosage forms, we say the patient should receive Y mL (milliliter), because mL is a unit of measurement reserved for liquids.

Concentrations are usually given in units of mg/mL. Hence, if the concentration is labeled as 10 mg/mL, we know there are 10 mg of a drug dissolved in every 1 mL of liquid.

Let's say our patient weighs 25 kg. The relative dose is given as 30 mg/kg. The first step is to calculate the amount we need in mg, just like for solids. So, we repeat exactly what we did in the last section:

  • 25 kg * 30 mg/kg = 750 mg.

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