The human body is incredibly complex
and the effects of a drug or medication can vary greatly between people
What works for one person
may not be as effective for others
or may cause side effects
This variation can be due to many factors, like differences in age
or size
your overall health
or interactions with other medications you might be taking
Another key factor that can affect how
you respond to a drug or medication
is variation in your genes
The study of the interaction between your genes and medications is called pharmacogenomics
An example of this is a gene called CYP2D6
CYP2D6 makes a protein responsible for breaking down many different medications
including the
painkiller codeine
When CYP2D6 breaks down codeine
it converts it into another molecule, morphine
which relieves pain
Some people have a variant in the CYP2D6 gene
that causes it to be less active
When these people, known as 
poor metabolisers, take codeine
it doesn't get converted to morphine as efficiently
This means that the medication may be
less effective at relieving pain
Other people have a variant in CYP2D6 that causes them to convert codeine to morphine too quickly
In these people, known as rapid or ultrarapid metabolisers
even normal doses of codeine can cause too much morphine to build up in the body
This means they might be at an increased risk of 
side effects
One of the aims of pharmacogenomics
is to understand whether people are poor or rapid metabolisers for certain medications
This information could help your doctor to prescribe the medications
that will work best for you, at the right dose, with less trial and error
Pharmacogenomic information can be relevant throughout a person's lifetime
Each time you need to take a new medication
your doctor could refer to your genetic information
to see if the medication might be less effective than
expected
or if you might be at an increased risk of side effects
