What to watch with CYP2D6 and allergy?

CYP2D6 is a key enzyme for metabolizing many drugs, including some antihistamines and codeine (used for allergy-related cough). When managing allergies, watch your CYP2D6 activity: Poor Metabolizers (PM) break down certain antihistamines like diphenhydramine or hydroxyzine more slowly, increasing side effects (drowsiness, dizziness). Ultrarapid Metabolizers (UM) risk converting codeine to morphine too quickly, potentially causing respiratory depression. Evidence is human-strong: CPIC and DPWG guidelines recommend genotype-based dosing for codeine and tramadol. For antihistamines, data are less robust, but caution is warranted. A genetic test provides insight but does not replace clinical advice. Drug interactions with CYP2D6 inhibitors (e.g., paroxetine, bupropion) can alter phenotype. Caveat: Many allergy drugs (e.g., cetirizine, loratadine) are minimally metabolized by CYP2D6, so the effect is not universal.