This phase 2B is designed to test the effectiveness of intranasal Oxytocin on Prader Willi Syndrome (PWS).

This is a prospective, multicentre, randomised, double-blind, Phase 2B clinical study planned to include around 24 PWS patients aged 2-17 years and 5 months.

Inclusion Criteria:

1. Male or female patients between 2 years and 17 years and 5 months at inclusion.
2. Genetically confirmed diagnosis of PWS.
3. Parents (or legal representative) have signed the informed consent form and are willing to comply with all study procedures.

Exclusion Criteria:

1. A history of hypersensitivity to the study drug or drugs with similar chemical structures, to excipients of the product, or to latex;

2. Intolerance of intranasal administrations (including when due to a major behavioural problem);

3. Hyponatremia (clinically relevant at the discretion of the investigator);

4. Hypokalaemia (clinically relevant at the discretion of the investigator);

5. Prolongation of the QT interval and/or family history of prolongation of the QT interval;

6. Concomitant treatment prolonging the QT interval;

7. Start of growth hormone (GH) treatment within the last 4 weeks before inclusion;

8. History of abnormal electrocardiogram (ECG) (validated by a cardiologist);

9. Pregnant girls; (for girls with childbearing potential who do not have contraception and are sexually active, a negative pregnancy test will be required)

10. Patient with clinical signs in the context of contact with COVID-19 infected person.

11. Patient included in another study protocol on a medicinal product within the last 6 months;

12. Administrative problems:

    1. Inability to give parents (or legal representatives) expert medical information;
    2. No coverage by a social security regime.