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Are You Prone To Side Effects?

Most medicines have side effects. But whether or not you will experience them and how long they last for depends on one thing….you!

Even though we all are human and look similar, there is a multitude of differences between us, many of which impact on the medicines we take.

Some of us are old and some are young. Some have kidneys that don’t work so well, while others have problems with other organs that change how drugs are processed by our bodies or excreted out of them. Some of us smoke and some drink too much. But nothing differentiates us more from each other than our genes. And when it comes to drugs and our genes, it’s time to sit up and take notice.

Genes are segments of DNA that have a function. They determine everything from our eye color to how tall we are, in addition to also influencing how we respond to drugs. If you want to blame your parents for something, you can blame them for your genes. Because all of us get one copy of each gene from good ol’ Mum and Dad.

Some experts estimate genetic factors account for up to 95% of patient variability. Let’s take codeine for example. Codeine is given to relieve pain. But for it to do its job, it must be metabolized into morphine, and that requires a liver enzyme called CYP2D6.

There are over 100 different forms of CYP2D6, ranging from non-functioning to extremely high functioning, with lots of variation in between.

An estimated 5 to 10% of the population have non-functioning forms of CYP2D6. These people are known as poor metabolizers. Because their liver is unable to convert codeine into morphine, they get little or no pain relief from the drug, but still potentially end up with side effects such as constipation.

At the other end of the spectrum are the ultra-fast metabolizers. These people carry at least 3 copies of the CYP2D6 gene and metabolize codeine into morphine at such a rate that even at normal dosages they are at high risk of an overdose and severe side effects such as extreme sleepiness, confusion, and shallow breathing. An estimated 28% of North Africans, 10% of Caucasians, and 1% of Hispanics over-convert codeine into morphine.

But codeine isn’t the only drug metabolized by CYP2D6. Many antidepressants, antipsychotics, analgesics, and beta-blockers are affected by it as well. If you take into account that there are 50 different types of hepatic enzymes, then you start to realize just why some people respond better or have fewer side effects to certain drugs than others.

Genetic testing will determine if you are a poor or extensive metabolizer of certain drugs, and you may like to consider it if you tend to be prone to drug side effects.

At, we group side effects into those that are more common, less common and rare. You can see this layout if you look at the side effects page of Aleve. This can help you to see if what you are feeling is typical for your new medication, and our tips pages give you some advice on what you can do about it. You may even like to look up your new drug before you start taking it so that you know what to expect.

Always remember that just because a side effect is listed as common, that doesn’t mean that you will always experience it.  But even if a medicine does make you feel sick or drowsy or not quite yourself when you first start taking it, chances are those effects will only be temporary, and get better with time.

For more information see Managing Common Drug Side Effects.

Posted: October 2018

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