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.

Remember that since she’s taking this medicine twice per day, she is actually taking 10 milliliters per day (Figure 5).

Next, we can divide 100 milliliters, which is the total amount in the bottle, by 10 milliliters per day, to get the number of days the medication will last, which is 10 days. (Figure 6).

Sometimes we need to find out how much of a product will be needed to provide a patient with a 30 day supply of a medicine.

So let’s say that there is a man who’s taking medication for seizures.

If he takes 2 tablespoons of medication three times a day, how many liters of does he need for a 30 day supply?

To figure this out, we first need to make sure that we are using consistent units of measure.

We know that 1 tablespoon is equal to approximately 15 milliliters.

If this man takes 2 tablespoons per dose, we know that he’s taking 30 milliliters per dose (Figure 7).

If he takes this medicine three times per day, we will then take 30 milliliters times 3 to get 90 milliliters per day (Figure 8).

Now that we know his total daily dose, we can multiply 90 milliliters by 30 days to get a value of 2700 milliliters per 30 days (Figure 9).

Lastly, we want to convert this number from milliliters to liters.

So we take 2700 milliliters times 0.001 or divided by a thousand, and we find out that he needs 2.7 liters of the seizure medicine to last him 30 days (Figure 10).

Now let’s work on a problem that combines weight-based dose and the amount needed for a certain number of days.

We have a woman who is 60 kilograms, and she is starting an antibiotic that is dosed at 30 milligrams per kilogram per day, and she will be taking this medicine for 10 days.

If the antibiotic comes in a concentration of 250 milligrams per 5 milliliters, how much of the antibiotic do you need to give her in milliliters?

We first need to figure out how many milligrams of the medicine she needs per day.

If we multiply her weight of 60 kilograms by 30 milligrams per kilogram per day, we know that she needs 1800 milligrams of the antibiotic every day (Figure 11).

If we have a concentration of 250 milligrams per 5 milliliters, we can set up a proportion where 250 milligrams over 5 milliliters is equal to 1800 milligrams over X milliliters.

We will then multiply 5 milliliters by 1800 milligrams to get a value of 9000, and divide this number by 250 milligrams to get an answer of 36 milliliters (Figure 12).

This means that the woman will need to take 36 milliliters of her medicine every day.