Fragile X Syndrome (FXS) is a neurodevelopmental disorder caused by a CGG expansion in the FMR1 gene located at Xq27.3. Patients with the premutation in FMR1 present specific clinical problems associated with the number of CGG repeats (55-200 CGG repeats). Premutation carriers have elevated FMR1 mRNA expression levels, which have been associated with neurotoxicity potentially causing neurodevelopmental problems or neurological problems associated with aging. However, cognitive impairments or neurological problems may also be related to increased vulnerability of premutation carriers to neurotoxicants, including phenobarbital. Here we present a study of three sisters with the premutation who were exposed differentially to phenobarbital therapy throughout their lives, allowing us to compare the neurological effects of this drug in these patients.