I’m glad you asked! The good news is that there are an increasing number of tests that can help a clinician chase down what is causing some mental health problems.
Genetic testing can help find out how your body processes medication. Why does that matter? Some medications have to be metabolized before they can be used by the body for their intended purposes. For example, codeine is metabolized in the liver by cytochrome P450 CYP2D6. CYP2D6 is an enzyme that is about 2% of the liver’s CYP system, but it processes about 25% of clinically used drugs (Wang et al, 2006). 2D6 is polymorphic, which means that it comes in a number of different forms as dictated by the DNA that codes for its construction. Some people have a low speed, some a medium speed, and some have a high speed version of 2D6. If you have a low speed 2D6, codeine may not be transformed into morphine at a rate that would provide you with pain relief. A medium speed 2D6 would metabolize codeine as the manufacturer expects. A high-speed 2D6 might convert codeine into morphine too quickly, and oversedate before it wears off much too early. Maybe codeine is not a good fit for a person with a high-speed 2D6!
That information alone is very useful, but it is even more useful in the context of a patient’s personal situation. There are substances that affect how 2D6 processes medication. For example, Prozac (fluoxetine) slows down 2D6. The importance of this is that changes in one medication can affect how another one works. “After I started taking Prozac my depression got better, but now my knees hurt just like before I started taking codeine. What happened?” Now we can find out.
There is a small group of people who have the right enzyme to make the best use of Zyprexa (olanzapine). They can use a low dose and get relief of their symptoms without running the risk of winding up with metabolic syndrome. The weight gain alone can cause people to stop taking that medication, but the diabetes that comes with metabolic syndrome can take years off a person’s life. As a prescriber, I want to avoid causing a person to go through that series of events. Genetic testing can help me avoid causing that particular problem.
Genetic testing is a great tool to have available. Ten years ago, it was simply not available. Psychiatrists had to work with trial and error to find medications that would work for patients. It’s a good thing that era has finally come to an end.
Wang, B., Yang, L., Zhang, X., Huang, S., Bartlam, M., & Zhou, S. (2009). New insights into the structural characteristics and functional relevance of the human cytochrome P450 2D6 enzyme. Drug Metabolism Reviews, 41(4), 573-643. doi:10.1080/03602530903118729