Inclusion Criteria:
* Signed Written Informed Consent a.Participants or their legal representative must have signed and dated an IRB/IEC approved written informed consent form
* Diagnosis of an Autism Spectrum Disorder according to the DSM-5 criteria
* Participation in the AIMS-2 CT1 (ages at recruitment 5 to 17).
* Current pharmacological treatment regimen affecting behaviour has been stable for at least 6 weeks prior to screening and is expected to be stable during the duration of the study
* Current psychotherapeutic/psychosocial interventions affecting behaviour stable for 3 months prior to screening and expected to be stable during the duration of the study
* Participants with a history of seizure disorder must currently be receiving stable treatment with anticonvulsant medication and must have been seizure free for 6 months prior to screening or must be seizure free for 3 years prior to screening if not currently on a stable (\>3 months) dose of antiepileptics
* Male or female participants 7 to 23 years of age at the time of providing consent, inclusive.
* Reside or regular contact (at least twice a week) with the parent/carer who is interviewed for the study.
* Negative pregnancy test for females of childbearing potential (participant has experienced onset of menses)
* Females of childbearing potential who are sexually active must agree to use a highly effective form of contraception (i.e., existing surgical sterilization, complete or abstinence or a combination of two affective forms of contraception, such as, for example, condoms plus hormonal treatment). Please, refer to Appendix 4 for a complete list of acceptable contraception methods.(protocol)
* Male participants with female partners of childbearing potential are eligible to participate if they agree to the conditions stated in section 8.2.1.(protocol)
Exclusion Criteria:
* Participants with any condition that might interfere with the conduct of the study, confound interpretation of the study results, or endanger their own well-being.
* Participants who are currently receiving treatment with racemic baclofen, vigabatrin, tiagabine, or riluzole or other GABA-related medications (e.g. gabapentin or pregabalin) other than arbaclofen in the context of AIMS-2 CT1
* Participants who are currently receiving pharmacologic treatment affecting behaviour (see concomitant medication section) need to have a stable dose during the 6 weeks prior to the screening visit and for the duration of the study.
* Participating in programs including non-pharmacologic educational, behavioural, and/or dietary interventions affecting behaviour, participation in these programs must have been continuous during the 3 months prior to screening and participants or their parent/caregiver/LAR may not electively initiate new or modify ongoing interventions for the duration of the study. Typical school vacations are not considered modifications of stable programming
* Participants who have taken another investigational drug within the last 30 days.
* Participants with evidence of any significant haematological, endocrine, cardiovascular (including uncorrected symptomatic congenital heart disease), respiratory, renal, hepatic, or gastrointestinal disease, not including mild common paediatric diseases in these areas that are stable (e.g. mild asthma, constipation, etc.), as judged by the investigator.
* Participants who are not able to take oral medications.
* Participants who have a history of hypersensitivity to racemic baclofen
* Participants with rare hereditary problems of galactose intolerance, the lactase deficiency or glucose-galactose malabsorption should not take this medicine.
* Active peptic ulceration as Baclofen stimulates gastric acid secretion.
* Porphyria.
* Participants who are currently engaged in illicit drug use or alcohol abuse, according to DSM-5 criteria.
* Participants who have previously participated in a clinical trial with arbaclofen (other than our AIMS-2-CT1).
* Women who are breastfeeding
