Calculating tablet doses

Tablet dose calculations

Often the medical assistant must use the drug order or the drug label and calculate the number of tablets needed. The following two sections describe how to calculate the number of tablets needed for two different scenarios - for the entire course of treatment and for a dose.

Number of tablets needed for entire course

It is important for the medical assistant to be able to calculate how many tablets a patient needs for an entire course of treatment. This section describes how to calculate the number of tablets needed for the entire course of treatment.

It is not uncommon for patients to call the provider to request enough medication for a trip. The same steps can be used if you need to calculate the number of tablets needed for a specific period of time.

A drug order such as Amoxicillin 500 mg, 1 tab tid po × 10 days breaks down into the following parts:

• Medication and tablet strength: Amoxicillin 500 mg
• Dose: 1 tab (tablet)
• Frequency: tid (three times a day)
• Route: po (oral)
• Duration: 10 days

Problem

Amoxicillin 500 mg, 1 tab tid po × 10 days. How many tablets would the patient need for the entire course of medication?

Solution for problem

Step 1: Figure out the number of tablets per dose. (Use the abbreviation list if you need to. One tab equals 1 tablet.)

Step 2: Figure out the number of tablets taken throughout the day. Do this by multiplying the number of tablets per dose (1 tablet) by the number of times the medication is taken during the day (tid means 3 times a day).

$$1\text{ tablet per dose}\times 3\text{ times a day}=3\text{ tablets per day}$$

Step 3: Figure out the number of tablets needed. Multiply the number of tablets per day by the number of days.

$$3\text{ tablets per day}\times 10\text{ days}=30\text{ tablets needed}$$

Number of tablets per dose

It is common for the provider to give medication orders without a tablet amount. For instance, the provider orders “Tylenol 975 mg po.” The medical assistant must figure out how many tablets to give the patient for each dose. To start, figure out the number of milligrams in each tablet, which will be indicated on the stock medication label. Once the tablet size is identified, the medical assistant can calculate the number of tablets required.

Using the proportion method

The following steps show how to calculate the number of tablets needed for the dose.

Problem

Order: Tylenol 975 mg po.

Stock: Tylenol 325 mg per tablet.

How many tablets should be given?

Solution for problem

Step 1: Put the stock information into a fraction on one side. It does not matter the location of the numbers. Make sure to add the labels. (Remember: 325 mg per tablet means 325 mg in each tablet.)

Step 2: Add an equal sign and make a fraction on the opposite side. Fill in the labels. The labels need to be in identical locations on both sides of the equal sign.

$$\frac{325\text{mg}}{1\text{tab}}=\frac{\text{mg}}{\text{tab}}$$

Step 3: Fill in the order information. You do not know the number of tablets, so that is the unknown (x).

$$\frac{325\text{mg}}{1\text{tab}}=\frac{975\text{mg}}{x\text{tab}}$$

Step 4: Now solve for the unknown using the proportion set up in Step 3.

Using a calculator: Multiply in the diagonal direction of the two numbers (1 × 975), and then divide by the number in the other direction (325); the answer is x.

$$1\times 975=975$$

$$975÷325=3$$

To solve without a calculator: Multiply diagonally and solve for x.

$$1\times 975=325x$$

$$975=325x$$

$$\frac{975}{325}=x$$

$$3=x$$

It is important to double-check your answers. If the order is larger than the stock tablet, then you will be giving more than 1 tablet. If the order is smaller than the stock tablet amount, then you will be giving less than a tablet.

Using the formula method

The formula method can also be used to solve tablet dose problems. The following example applies it to a new order.

Problem

Order: Lisinopril 15 mg po.

Stock: Lisinopril 10 mg per tablet.

How many tablets should be given?

Formula method setup

The setup for the formula method is the following:

$$\frac{D(\text{desired [order]})\times V(\text{vehicle [tablet or liquid]})}{H(\text{on hand})}$$

Step 1: D (desired) is 15 mg. H (on hand) is 10 mg. V (vehicle) is 1 tablet. Put the values into the formula method.

$$\frac{15\text{mg}\times 1\text{tablet}}{10\text{mg}}$$

Step 2: Multiply 15 by 1 and then divide the answer (15) by 10 to figure out the tablet amount.

$$\frac{15\text{mg}\times 1\text{tablet}}{10\text{mg}}=\frac{15}{10}$$

This chapter covered tablet doses; the next chapter, Calculating liquid and solution doses, covers how to apply the same proportion and formula methods to liquid medications.