Type 1 Gaucher disease is a rare genetic disease characterized by enzymatic deficit leading to glucosylceramide overload in body tissues (lysosomal overload disease). Standard treatment is based on substitutive enzyme therapy by intravenous perfusion. A new drug for oral administration, eliglustat, was recently awarded marketing approval in Europe and the USA. Eliglustat acts by reducing the enzyme substrate. Eliglustat is mainly eliminated by a CYP2D6 pathway. CYP2D6 exhibits genetic variability or expression, leading to 20-fold differences in serum levels. In marketing approval documents, both the FDA and the EMA mention the requirement for CYP2D6 genotyping before prescribing eliglustat: the drug is contraindicated for ultra-rapid metabolizers (under-dosing inefficacy) and slow metabolizers should be given a 50% reduced daily dose (risk of overdose-related adverse effects). Finally, potential drug interactions (inhibition or induction of CYP2D6 or CYP3A40) are also dependent on CYP2D6 genotyping, such that prescribers must be aware of a patient's genotype before prescribing eliglustat.