Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease with limited therapeutic options. Riluzole remains the only widely available disease-modifying treatment for ALS, yet its survival benefit is modest and likely to vary substantially between patients. Cytochrome P450 2D6 (CYP2D6), is a highly polymorphic enzyme that contributes to interindividual variability in the metabolism of many drugs. CYP2D6 is also expressed in the brain, and experimental and translational studies indicate that brain CYP2D activity can influence local metabolism of neuroactive compounds. Accordingly, CYP2D6 poor function variants have been examined as susceptibility modifiers in the development of other neurodegenerative diseases, including Parkinson's disease and Alzheimer's disease, with heterogenous evidence; however, the role of CYP2D6 in ALS has not been established.