A compound represented by the structural formula I (GENZ 112638, eliglustat tartrate), which may be used in pharmaceutical applications, is disclosed. Particular single crystalline forms of the compund are characterized by a variety of properties and physical measurements. As well, methods of producing crystalline forms of the compound, and using it to inhibit glucosylceramide synthase or lowering glycosphingolipid concentrations in subjects to treat a number of diseases such as Gaucher disease and Fabry disease, are also discussed. Also described is the treatment of Fabry disease in a subject who has been assessed as being a poor, intermediate or extensive P450 metabolizer.
