The FENRTT2 study will investigate the efficacy and safety of a medicinal cannabis plant extract with extremely low THC (delta-9-tetrahydrocannabinol), NTI164, on Rett syndrome (RTT) in a crossover design. RTT is a devastating rare genetic condition affecting females and involves debilitating physical and intellectual symptoms. NTI164 is an oil which has demonstrated efficacy in reducing symptoms in several paediatric neurological conditions, including RTT, autism spectrum disorder (ASD), and paediatric acute-onset neuropsychicatric syndrome (PANS). A Phase I/II clinical trial of NTI164 in RTT (FENRTT1/NTIRTT1) showed NTI164 is safe in this population and significantly improved overall clinical severity of illness, as well as core RTT symptoms, including anxiety, mental alertness, communication skills, socialisation/eye contact, and attentiveness. The FENRTT2 study will investigate NTI164 in a larger number of patients, and compare NTI164 to a placebo control. Research tests on patient blood will also be included to further investigate how NTI164 works in the body.