Skip to main content

Do genes determine your response to drugs?

By Jason Poquette, BPharm, R.Ph

Brad Pitt reportedly claimed to be “one of those people you hate because of genetics.” I’m not sure about that, but I do know that genetics determine much more than our good looks or acting talent.  In fact, they have a profound impact on the way individual patients respond to drugs. This field of study is known as pharmacogenomics, and understanding the unique genetic characteristics of a patient should sometimes influence how and what we prescribe.

Your particular genes can make a drug which is entirely safe and effective for most people, dangerous and even fatal for you. An historical example is the well-known aversion that Pythagoras (570-495 BC) had for fava beans. While there are many rumors and stories about this, one theory is that he recognized the danger these beans posed for certain individuals. We now know that indeed some people have a generic trait which leads to a deficiency in the production of glucose-6-phosphate dehydrogenase (G6PD deficiency, also known as favism). Such individuals, if they consume fava beans (and certain other foods) will develop hemolytic anemia, a potentially life-threatening condition.

More recently, and more commonly, we have been growing in our awareness that the use of codeine in children carries significant risks, depending on their genetic makeup. In a recent report from the American Academy of Pediatrics, doctors are increasing their vigilance in warning about the risks associated with codeine in some patients. The article, entitled “Codeine: Time to Say ‘No’” reminds readers that “drug surveillance has documented the occurrence of unanticipated respiratory depression and death after receiving codeine in children.” 

The tragic and often unpredictable response is due to genetics. Our bodies metabolize codeine into morphine in order for it to exert an analgesic (pain relief) effect. Normally this is fine, but for some patients with a genetic variation leading to higher than normal amounts of this metabolizing enzyme (known as CYP2D6) their bodies produce much larger amounts of morphine. The result is basically the same as receiving an overdose, even though taking an otherwise normal amount. 

The FDA has been warning about this for years in certain patients (after getting a tonsillectomy for example), but still there have been thousands of prescriptions written for codeine for children in recent years. In fact, a 2011 study reported that codeine was prescribed more than 800,000 times for patients under the age of 11. 

But genetic responses aren’t only a concern for kids and codeine. As research continues we are becoming increasingly aware of more medications that, depending on genetic factors, may be either ineffective or dangerous for certain patients. A common example is the drug omeprazole.  Available without a prescription, omeprazole is commonly taken to help reduce acid production in patients with GERD. Some patients, however, have genetically determined variations in the way their body metabolizes this drug and it can lead to either higher or lower than expected blood levels.

Genetic variations in responses to a drug was even the subject of a lawsuit a couple years ago. The makers of an anti-platelet drug known as Plavix® (clopidogrel) were accused of deceptive marketing their drug in Hawaii, an area known for having a high concentration of people for whom the drug would be genetically ineffective.   

In spite of all this, routine gene testing of patients is rare. For now, we are best served by knowing that our genetics really can make a big difference in how we respond to drugs. And while we shouldn’t hate anyone for the genes they inherited, we might just want to prescribe differently for them.