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Taking the trial and error out of prescribing medications

Jill Furnival

A new genetic test is able to tell a woman if she is in the 1-2% of people who should avoid taking the pain medication codeine, especially while breastfeeding. It is common for women to need some type of pain relief after delivery. However, for some women, taking codeine while breastfeeding can cause a life-threatening level of the medication in her breast milk, putting her newborn at risk of an overdose.

According to Jama Network, the risk depends on how her body processes the drug. And in recent years, scientists have revealed how we process codeine, and several other medications, is determined in part by our genes.

When we take codeine, it enters our body in an inactive form. In order for codeine to do its job (i.e. provide pain relief), an enzyme called CYP2D6 needs to convert it into morphine. Based on variants in our DNA, different people make different amounts of CYP2D6, which impacts how we metabolize the drug. People who make more CYP2D6 are ultrarapid metabolizers of codeine, which means they convert codeine into morphine faster than others. The result is higher than expected levels of morphine in the body, and for a breastfeeding mother, in her breast milk.

Understanding the genetics behind how people metabolize drugs is part of a new and emerging field called pharmacogenomics. Experts are developing genetic tests that will be able to predict a person’s response to certain drugs, with the goal of reducing the traditional ‘trial and error’ tactic to prescribing medications.

Novel pharmacogenomic test panels can tell you if you are an ultrarapid metabolizer, rapid metabolizer, normal metabolizer, intermediate metabolizer, or poor metabolizer for a long list of drugs. And knowing this can help your doctor chose the right drug and even the right dose for you. Which means you get better faster and face a lower risk for unpleasant, or potentially dangerous, side effects. At least that’s the plan.

Experts have now identified more than 300 medications influenced by pharmacogenomic factors, and among those are many commonly prescribed medications such as antidepressants, blood thinners, chemotherapies, pain relievers, and cholesterol-lowering statins.

But doctors have been slow to adopt the testing into everyday practice, as they need guidance on when to order it and how to understand and use the results. That’s why there are on-going educational initiatives in the US aimed at getting pharmacogenomics into the clinic.

It is important to say that pharmacogenomic testing does not provide all of the answers; there are many factors that influence how we respond to medications (e.g.: body weight, gender, other medications). It does not replace the clinical judgment of your doctor. Rather, it adds to it.

The thought is; pharmacogenomic testing is a great place to start. It can help doctors avoid prescribing medications that are expected to have toxic effects, keeping people safer, and guide them towards the medications and dosages that are expected to work, relieving symptoms faster.

Experts believe it won’t be long before we look back on the longstanding ‘trial and error’ approach to prescribing medications as a way of the past.

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