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Welcome back to the RelayEd Pharmacy Student Toolkit, a practical study newsletter designed to help pharmacy students build confidence one skill at a time.

Each issue breaks down a pharmacy concept you may encounter in pharmacy school, on the NAPLEX, or in practice, with clear explanations, worked examples, and quick practice to help reinforce what you learn.

Today’s Theme: Nonsterile Compounding Terms You Need to Know

In class, in the lab, and on the NAPLEX, you'll need to recognize nonsterile compounding techniques and terms.

A few terms sound similar enough that they are easy to mix up:

API. Excipient. Vehicle. Levigating agent. Suspending agent. Emulsifying agent. Solution. Suspension. Emulsion.

In this issue, we'll define and explain these.

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Start Here: API vs. Excipient

Every compounded nonsterile preparation contains one or more active pharmaceutical ingredients (APIs) and may contain one or more excipients (AKA added substances).

API

The active pharmaceutical ingredient is the ingredient responsible for the intended pharmacologic effect.

Think:

API = the drug doing the therapeutic work

Excipient

An excipient is an ingredient included in the preparation for a specific formulation purpose rather than for the intended pharmacologic action.

Excipients can help:

• Create the desired dosage form
• Improve stability
• Improve solubility
• Improve taste or appearance
• Support drug delivery
• Change viscosity
• Help distribute the API throughout the preparation

Importantly, “inactive” does not mean “irrelevant.” Excipients still need to be appropriate for the patient, route of administration, and formulation. They can also cause real side effects and issues for patients if not selected carefully.

Quick memory trick

API = what the drug DOES

Excipient = helps the FORMULATION work

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Compounding Vocabulary: What Is Each Ingredient Doing?

Vehicle

A vehicle is the base or medium used to carry the ingredients in a preparation.

Think:

Vehicle = what carries the drug

For example, a liquid medication may use an aqueous vehicle.

Levigating agent

A levigating agent is a liquid used to help reduce the particle size of a powder during grinding and help incorporate that powder into a preparation.

Examples include:

• Mineral oil
• Glycerin
• Propylene glycol

Think:

Levigation = smooth out the powder

This is especially useful when incorporating a powder into a semisolid preparation.

Suspending Agent

A suspension contains solid particles that are not dissolved in a liquid.

A suspending agent helps slow the settling of those particles by increasing viscosity or otherwise helping maintain dispersion.

The particles can still settle over time, which is why suspensions generally need to be shaken before use.

Think:

Suspending agent = helps keep solid particles suspended

Emulsifying Agent

Oil and water normally do not mix well.

An emulsifying agent helps maintain the dispersion of one immiscible liquid within another.

That becomes important in an emulsion, where one liquid is dispersed as droplets within another liquid.

Think:

Emulsifier = helps two liquids that don't normally mix stay together

Gelling Agent

A gelling agent increases viscosity and helps form the structured network of a gel.

Common examples include ingredients such as:

• Carbomer
• Methylcellulose
• Sodium alginate
• Xanthan gum

Know the Difference: Solution vs. Suspension vs. Emulsion

This is an important compounding distinction to know.

Solution

Drug completely dissolved in the vehicle

There should not be visible undissolved particles of the drug.

Suspension

Solid dispersed in a liquid

The drug is not completely dissolved.

Particles can settle → SHAKE WELL

Emulsion

Liquid dispersed in another immiscible liquid

Think:

oil + water + emulsifying agent

Try It Yourself

A technician is preparing a compounded oral medication.

The active drug does not dissolve in the liquid vehicle. Instead, small solid drug particles remain dispersed throughout the liquid.

Which dosage form is being prepared?

A. Solution
B. Suspension
C. Emulsion
D. Ointment

Answer: B Suspension

The key phrase is:

solid particles remain dispersed in a liquid

That describes a suspension.

Because those solid particles can settle over time, the preparation generally needs to be shaken before administration to redistribute the particles and help ensure accurate dosing.

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Compounding Spotlight: Levigation

Here is another term worth recognizing.

Imagine you are adding a powdered drug to an ointment.

Simply dumping the powder into the ointment base may leave the final product gritty or uneven.

Instead, the powder can be worked with a small amount of an appropriate levigating agent.

The goal is to reduce particle size and create a smoother mixture before incorporating it into the remaining base.

Do not confuse:

Levigation​
→ focuses on reducing particle size and helping incorporate a powder into a preparation

Suspending agent​
→ helps reduce settling of particles in a suspension

Emulsifying agent​
→ helps maintain dispersion between immiscible liquids

These ingredients and techniques solve different formulation problems.

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Compounding Pearl

A formulation term often tells you what problem an ingredient is solving.

Instead of memorizing a giant list, ask:

What is this ingredient supposed to accomplish?

• Drug keeps settling? → Suspending agent
• Oil and water need to stay dispersed? → Emulsifying agent
• Preparation needs to be thicker? → Viscosity-increasing or gelling agent
• Powder needs to be worked smoothly into a semisolid? → Levigating agent
• Preparation needs a medium to carry the drug? → Vehicle

That makes the vocabulary much easier to remember.

Safety Check: Does the Preparation Look Right?

Appearance matters in compounding.

Potential signs of instability can include:

• Color change
• New or unusual odor
• Precipitation or crystallization
• Phase separation
• Change in viscosity
• Gas formation

If instability is suspected, the preparation should be evaluated rather than simply assuming the change is normal.

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One-Minute Knowledge Check

1.Which term describes the therapeutically active ingredient in a compounded preparation?

A. Vehicle
B. API
C. Levigating agent
D. Excipient

2. Which preparation contains solid particles dispersed in a liquid?

A. Solution
B. Suspension
C. Emulsion
D. Gel

3. Which ingredient helps maintain dispersion of one immiscible liquid within another?

A. Emulsifying agent
B. Levigating agent
C. Sweetening agent
D. Diluent

Answers below.

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Liz Fredrickson, PharmD, BCPS Founder, RelayEd liz@relayedllc.com

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