Pharmacy school: Dosage calculations

Dosage calculations are used to figure out the right amount of medicine to give a patient based on that particular patient’s needs.

Different things can affect a patient’s dosage requirements of a medicine, including age, weight, laboratory values, or it may even depend on the medicine itself.

Some calculations can be simple, while others require conversions and use of proportions.

This concept can be a bit intimidating at first, but with a little practice, calculating doses will become second nature.

Let’s start out with some basic examples of calculations that you might encounter.

Let’s say that we have a 140 kilogram person who needs a specific medication for an infection.

The dosage for this medication is 6 milligrams per kilogram per dose.

What dose would this patient require in grams?

We would start out by taking the person’s weight which is 140 kilograms and multiply it by 6 milligrams per kilogram to get 840 milligrams per dose (Figure 1).

However, we aren’t quite done.

To convert milligrams to grams, we know that for every 1 milligram of a product, this is equal to 0.001 grams of the same product.

Since we know that this patient would need 840 milligrams of the medicine, we can then take 840 milligrams and multiply this number by 0.001, which is the same as dividing by a thousand, to get an answer of 0.84 grams (Figure 2).

Now, there may be times when you or someone else has already selected a dose for a patient, but you need to figure out what that dose is per kilogram of the patient’s weight.

Let’s say that you receive a prescription for a man who’s starting a medicine to treat a heart condition.

This medicine comes in a concentration of 40 milligrams per milliliter, and he’s supposed to take 1 milliliter daily.

If he weighs 70 kilograms, what dose is he getting in milligrams per kilogram?

We will first want to set up a proportion to figure out how many milligrams he is getting per dose.

If we set up an equation that 40 milligrams over 1 milliliter is equal to X milligrams over 1 milliliter, we know that the man is getting 40 milligrams per dose (Figure 3).

To determine how many milligrams per kilogram he is getting, we would then divide 40 milligrams by 70 kilograms to get our answer of 0.57 milligrams per kilogram per dose (Figure 4).

Dosage calculations also involve figuring out how many days a certain product will last for a patient.

For example, let’s say that we have a woman who buys a cough and cold medicine that has a concentration of 900 milligrams per 9 milliliters, and this medicine comes in a bottle that has 100 milliliters total.

She plans to take 1 teaspoon of this medicine two times a day.

How many days will this bottle last her?

We will first need to figure out how many milliliters she is taking of this medication per day.

We know that 1 teaspoon is equal to approximately 5 milliliters.

So if she’s taking 1 teaspoon per dose, this is equal to 5 milliliters per dose.