Although miglustat has been approved as a treatment for mild to moderate type 1 Gaucher disease in patients who are unsuitable for enzyme replacement therapy (ERT), more data are required to establish the long term efficacy, safety and tolerability of miglustat in maintaining diseases stability after a switch from ERT.

Inclusion Criteria:

1. Males or females aged 18 years or older

2. Type 1 Gaucher disease, diagnosed by glucocerebrosidase assay or molecular analysis of the glucocerebrosidase gene.

3. Treatment with ERT for at least 3 years, with a stable dose regimen for at least the last 6 months.

4. Clinically and biologically stable disease for the previous 2 years, with at least 2 time points assessments (including baseline as one potential time point), defined as:

   • Stable organomegaly (assessed by magnetic resonance imaging (MRI) or computed tomography (CT)):

     • Liver volume within 10% of the mean.
     • Spleen volume within 10% of the mean.

   • Free of progressive symptomatic documented bone disease.

   • Hemoglobin levels > 11g/dl

   • Mean platelet count > 100x10^9 /l.

   • Chitotriosidase activity within 20% of the mean.

     • If chitotriosidase is not available (in the case of chitotriosidase deficiency, or if it was not determined), other relevant biomarkers (e.g., angiotensin converting enzyme (ACE), tartrate resistant acid phosphatase (TRAP) and ferritin) could be considered.

5. Written informed consent.

Exclusion Criteria:

1. History or evidence of oculomotor gaze palsy, ataxia or other clinical manifestations typically associated with neuronopathic type 3 Gaucher disease.
2. Not ambulant patients, or with progressive symptomatic documented bone disease.
3. Splenectomy before 18 years of age for splenomegaly and/or thrombocytopenia.
4. Peripheral polyneuropathy (not mononeuropathy) documented with both clinical signs and symptoms, and electrodiagnostic (EDX).
5. Patients (males and females) who do not agree to use reliable contraception throughout the study and for 3 months after cessation of miglustat treatment.
6. Female patients who are pregnant or breast feeding, or without pregnancy test prior to Day 1.
7. History of significant lactose intolerance.
8. Clinically significant diarrhea (>3 liquid stools per day for >7 days) without definable cause within 6 months prior to Day 1, or a history of clinically relevant gastrointestinal disorders.
9. History of cataracts or known increased risk of cataract formation.
10. Severe renal impairment i.e., with a creatinine clearance <30 ml/min/1.73m^2
11. Concomitant active medical condition such as human immunodeficiency virus (HIV) or hepatitis B/C that would render patients unsuitable for study.
12. Previous treatment with miglustat.

From study treatment start to the study treatment end date plus 2 days