What Is Pharmacogenomics (PGx)? How Your Genes Can Affect Medication Response

Pharmacogenomics, often shortened to PGx, looks at how a person’s genes may influence the way they respond to medications.

People are genetically different. Because of these differences, the same medication and the same dose can work very well for one person, cause side effects in another, and provide little benefit to someone else.

PGx testing looks for some of these genetic differences to provide additional information that may help a healthcare professional choose a medication and dose that are more compatible with the individual patient.

A simple way to think about it is:

1. Your genes
2. Influence how your body handles and responds to medication
3. Medication may work differently from person to person

What Are Genes?

Genes are small sections of DNA that contain instructions used by the body.

You can think of genes as:

Instructions in your body’s operating manual

Different genes provide instructions that help control things such as:

• How medications are broken down
• How quickly medications leave the body
• How receptors in the brain and body function
• How medications interact with those receptors
• How sensitive someone may be to certain medication effects Everyone has essentially the same collection of genes, but the instructions within those genes can vary slightly from person to person.

These small differences help make each person biologically unique.

What Are Genetic Variations?

A genetic variation is a small difference in DNA.

An easy way to think about it is:

We all have the same instruction book, but some of the words or letters can be slightly different.

Most genetic variations are completely normal.

However, some variations can change how strongly a gene works or how efficiently the body performs a particular function.

For example, a genetic variation may cause someone to:

Break down a medication quickly

or:

Break it down slowly

Another variation may affect how strongly a medication interacts with a receptor in the brain.

These differences can help explain why two people taking the same medication can have very different experiences.

How Can Genes Affect Medication?

Genes can influence medication in two main ways.

How Your Body Handles the Medication

Some genes control the enzymes that break down medications.

Imagine the body has a series of:

Medication processing systems

Some people naturally process a particular medication:

Very quickly

while others process it:

More slowly

If a medication is removed too quickly:

There may not be enough medication available to work effectively.

If it is removed too slowly:

The medication may build up and increase the chance of side effects.

PGx testing can sometimes identify these differences before or during treatment.

How Your Body Responds to the Medication

Even when two people have the same amount of medication in their bodies, they may still respond differently.

Why?

Because medications work by interacting with biological targets such as:

• Receptors
• Transporters
• Enzymes
• Signaling systems Genes help determine how many of these targets are produced and how they function.

Therefore:

• Same medication
• Same dose does not necessarily mean:

Same response

One person may respond very well, while another may experience side effects or little improvement.

What Does Drug Compatibility Mean?

When we describe a medication as being: More genetically compatible

we do not mean that genetics can guarantee that the medication will work.

Instead, it means the person’s genetic profile may be more favourable for how that medication is processed or how its biological pathway functions.

A more compatible medication may have:

• More appropriate metabolism
• Lower likelihood of excessive or inadequate drug exposure
• A biological mechanism that appears more favourable for that individual
• Fewer known genetic concerns
• A potentially better balance between expected benefit and side-effect risk Think of it as:

Finding medications that appear to be a better biological fit for the individual.

What Does Drug Response Mean?

Drug response refers to:

What actually happens when a person takes a medication.

A good response may mean:

• Symptoms improve

• The medication is well tolerated

• Side effects are manageable

• The dose provides appropriate benefit A poor response may mean:

• Little or no improvement

• Significant side effects

• The medication works only partially

• A very high or very low dose appears necessary

• Treatment has to be stopped Genetics can contribute to these differences, but it is only one part of the picture.

Why Can the Same Medication Work Differently for Different People?

Consider two people who both take the same medication at the same dose.

Person A

Processes the medication normally and has a biological system that responds favourably.

Result: Good improvement with manageable side effects

Person B

Processes the medication slowly.

Result: Higher medication levels and more side effects

Person C

Processes the medication very quickly.

Result: Medication levels may be too low for a good response

Person D

Processes the medication normally but their biological target responds differently.

Result: The medication may still provide little benefit

This is why:

Normal medication metabolism does not always mean the medication will work well.

Both medication processing and medication response can matter.

What Can a PGx Test Tell You?

Depending on the medication and the genes tested, pharmacogenomic testing may provide information about:

• How quickly you may process a medication
• Whether the medication may build up in your body
• Whether you may need a different dose
• Whether certain side effects may be more likely
• Whether another medication may be worth considering
• Whether the biological pathway targeted by a medication appears more or less favourable The amount and strength of evidence varies from one medication and gene to another.

For some medications, genetic prescribing guidelines are well established.

For others, genetic information may provide additional clinical insight but is not strong enough to determine treatment on its own.

Can PGx Testing Tell Me Which Medication Will Work Best?

Not with certainty.

PGx testing can improve the information available when choosing a medication, but it cannot predict every aspect of medication response.

Many things influence treatment, including:

• Diagnosis
• Symptoms
• Previous medication response
• Other medications
• Age
• Kidney and liver function
• Other medical conditions
• Lifestyle
• Genetics PGx is therefore best thought of as:

An additional tool for personalized medication selection

rather than a test that guarantees the perfect medication.

Can PGx Testing Predict Side Effects?

Certain genetic variations are associated with:

• Higher medication levels

• Lower medication levels

• Increased sensitivity to particular drugs

• Increased risk of specific adverse effects When strong evidence exists, this information may help a healthcare professional:

• Use a lower dose

• Increase the dose more carefully

• Monitor more closely

• Choose another medication But genetics cannot predict every side effect.

What Is Medication Metabolism?

Medication metabolism simply means: How your body processes and removes a medication.

Your liver contains enzymes that act like Medication-processing machines.

Genes help determine how well some of these enzymes work.

Depending on your genetics, you might process a particular medication:

• Very slowly
• Somewhat slowly
• Normally
• Quickly
• Very quickly This can change how much medication remains in your body.

What Is a Poor Metabolizer?

A Poor Metabolizer has little or very low activity from a particular drug-processing enzyme.

For some medications:

1. Poor metabolism
2. Medication is removed more slowly
3. Medication levels can become higher
4. Side effects may become more likely

For other medications that need to be activated by an enzyme, the effect can be different.

This is why each medication must be interpreted individually.

What Is a Rapid or Ultrarapid Metabolizer?

A Rapid or Ultrarapid Metabolizer has increased activity from a particular drug-processing enzyme.

For some medications:

1. Fast metabolism
2. Medication is removed quickly
3. Drug levels may become too low
4. The medication may be less effective

Again, the effect depends on the medication because some drugs are converted by enzymes into active metabolites.

What Is Pharmacokinetics — PK?

A simple definition is: PK = What your body does to the medication

This includes:

• Absorption
• Metabolism
• Drug levels
• Elimination Genetic tests of drug-processing enzymes often provide information about:

Pharmacokinetics

For example:

1. Medication
2. Your genetic metabolism
3. How much medication reaches the bloodstream and brain

What Is Pharmacodynamics — PD?

A simple definition is: PD = What the medication does to your body

Once a medication reaches its target, it has to interact with:

• Receptors
• Transporters
• Enzymes
• Other biological systems Genes can influence these systems too.

Therefore:

1. Medication reaches the brain
2. Medication interacts with its target
3. Your biological system responds

This is:

Pharmacodynamics

Why Do Both PK and PD Matter?

Imagine a medication reaching the brain at exactly the expected concentration.

That tells us: The medication level may be appropriate.

But it still does not tell us whether: The patient’s brain will respond favourably to that medication.

Therefore:

PK asks: Is the right amount of medication reaching the body and brain?

while:

PD asks: Is this medication’s biological action a good fit for this patient?

Both may influence medication response.

A Simple Example of Pharmacogenomics

Imagine three people taking the same antidepressant.

Person 1

Processes the medication normally and responds well.

**Result:**Medication works well.

Person 2

Processes the medication very slowly.

**Result:**Medication levels become high and side effects develop.

Person 3

Processes the medication normally, but their biological response to that medication is less favourable.

**Result:**Normal medication levels but little improvement.

This is why personalized prescribing may need to look beyond:

“Which medication is normally used for this diagnosis?”

and ask:

“Which medication appears most appropriate for this particular person?”

What Does a PGx Report Mean When a Medication Is Favourable?

A favourable result usually means the genetic information reviewed does not identify major concerns and may show biological characteristics that support use of the medication.

It does not mean:

The medication is guaranteed to work.

A better way to interpret it is:

The patient’s genetic profile appears reasonably compatible with this medication based on the information tested.

What Does a PGx Report Mean When a Medication Requires Caution?

A caution result may mean that genetics suggest:

• The medication could reach unusually high levels
• The medication could reach unusually low levels
• A different dose may be appropriate
• Certain side effects may deserve greater attention
• The medication’s biological pathway may appear less favourable
• Another medication may be worth considering A caution does not automatically mean:

“Do not use this medication.”

It means:

There is genetic information the prescriber should consider when deciding whether and how to use the medication.

Does Everyone Have Genetic Variations?

Yes.

Genetic variation is normal.

Every person has millions of small DNA differences.

Most have no harmful effect.

Some, however, influence:

• Drug metabolism
• Receptor function
• Transporter activity
• Sensitivity to medications Pharmacogenomic testing focuses on variations that may be relevant to medication treatment.

Is Pharmacogenomic Testing the Same as Genetic Testing for Disease?

No.

PGx testing is primarily intended to understand:

How your genetics may affect medication treatment

rather than to determine whether you will develop a particular disease.

The main question is:

“How might this person’s biology influence the way they process or respond to this medication?”

Does Your Genetic Profile Change Over Time?

Your inherited genetic profile generally remains the same throughout your life.

This means a pharmacogenomic result can often remain useful when considering medications in the future.

However, your actual medication response can still change because of:

• Age
• Other medications
• Kidney or liver function
• Health conditions
• Hormonal changes
• Lifestyle A genetic result therefore remains important, but it must always be interpreted in the context of your current health.

The Bottom Line

Pharmacogenomics — or PGx — uses information from your genes to help understand why medications may affect you differently from someone else.

In simple terms:

1. Genes
2. Contain your biological instructions
3. Genetic variations
4. Create small differences between people
5. These differences can affect:

How medication is processed

and:

1. How the body responds to medication
2. Different people can respond differently to the same medication

PGx testing can help identify medications that may be more compatible with your individual biology, medications that may require dose adjustments or closer monitoring, and medications for which another option may be worth considering.

The goal is not to predict treatment perfectly.

The goal is to provide:

More information before choosing or adjusting a medication, so treatment can be better personalized to the individual rather than relying on trial and error alone.