Compounds

Compounds are medications made for a specific patient. Compounding involves multiple medications or a medication-type conversion such as taking a solid (tablets) to a liquid. The latter is popular for patients that cannot swallow solids well or are tube-fed; additionally, there is not a commercially-produced alternative so a compound is the only option.

Compounds are made into any form: topicals, liquids, intravenous, etc. Compound recipes can vary widely; Magic Mouthwash probably being the most common example of a compound with seemingly endless options. In order to be competent in a compounding pharmacy, you need to be able to calculate concentrations and percentages.

Concentration

A concentrated medication is a specific strength dissolved in a solvent. Commonly, measured in milligrams per milliliter (mg/mL). For example, sertraline, the generic for Zoloft, is available commercially as tablets in three strengths and liquid form in one strength, 20mg/mL.

Compounding Concentration Calculation Example

Let’s look look at an example where there is not currently a commercially-available liquid strength, acetazolamide, the generic for Diamox.

Zora has difficulty swallowing. Her doctor has prescribed her one acetazolamide 125mg tablet daily. She takes her prescription to her local compounding pharmacy so that she can get a liquid version. The compounding pharmacy will make this in a 25mg/mL 60mL suspension. How many 125mg tablets will this compound need?

First, calculate the total number of milligrams needed.

x = 1,500mg acetazolamide needed

Each tablet is 125mg. Divide the total milligrams needed by 125mg to get the number of tablets required to make this compound.

1,500 / 125 = 12 tablets

The compounding pharmacy will crush and mix twelve 125mg acetazolamide tablets to make this 25mg/mL suspension. Zora will need to consume 5mL of the suspension to equal the original prescription.

Percentage Calculation Example

Percentage is also the concentrated amount. Instead of seeing a compound written as mg/mL or g/mL, it is expressed as a percent like the intravenous fluid, dextrose 5%. The percentage of active drug will always be based on 100g or mL. For example, dextrose 5% means that there is 5g of dextrose for every 100mL of water.

Review how to calculate percentages, if necessary.

If a test question is how many grams of dextrose are in a dextrose 5% 500mL IV bag, you could set it up as a proportion; however, just some quick multiplication will do the trick. If there are 5g for every 100mL, then 500mL is going to be 25g.

Dilution

Dilution is decreasing the concentration of a solution. Generally, this technique is used for stock solutions and reconstitution of injectables e.g. Zostavax. A diluent such as sterile water is added to the drug to create the desired concentration.

Suppose we have 200mL of a 50% solution that needs to be diluted to 300mL. What will the percent strength be after dilution?

First, recall from the percentages lesson above that 200mL of a 50% solution translates to 50g per 100mL. Given that, we can set up the proportion.

100x = 10,000

x = 100g

Now, we see that 100g is diluted in 200mL so we can calculate the new percent strength.

300x = 10,000

x = 33.33%

The final calculation shows that when the dilute is increased from 200mL to 300mL, the concentration decreases from 50% to 33.33%.