Inclusion Criteria:  Male or female Aged over 21 Willing and able to comply with protocol Able to read, comprehend and record information written in English Capable of giving written informed consent, which includes compliance with the requirements and restrictions listed in the consent form. Healthy as determined by a responsible physician, based on a medical evaluation which includes medical history, a physical examination, laboratory tests (if required), and a psychiatric evaluation. A volunteer with clinical parameters outside the reference range for the population being studied may be included, only if the investigators concur that the finding is unlikely to jeopardize either subject safety or study integrity. DSM-5 diagnosis of current severe opioid use disorder Treated with methadone substitution therapy and able to maintain the same stable dose for screening and experimental visit. Ability to receive an acute dose of up to 90mg baclofen or up to 4800IU vitamin D (placebo).  Exclusion Criteria:  Intoxication on any of the visits, as assessed by difficulty in walking, the slurring of speech, difficulty concentrating or drowsiness. This exclusion criteria would exclude a subject from that study day only and not the whole study, at the discretion of the research team. Positive urine drug screens or breath alcohol at screening or experimental testing visits. A minimum list of drugs that will be screened for include amphetamines, cocaine, opiates, methadone, cannabinoids and benzodiazepines. Positive results for methadone will be allowed for those opiate dependent participants still undergoing OST. Positive results for cannabinoids will be allowed given the long half-life of cannabinoid metabolites. This exclusion criteria would exclude a subject from that study day only and not the whole study, at the discretion of the research team. Current DSM-5 substance dependence disorder for any other substance except for opiates and nicotine. Lifetime history of dependence on other substances will be allowed given very high incidence of co-dependence. Regular on-top use of heroin or other opiates or other illicit substances in combination with OST, which in the opinion of the investigators will interfere with subject safety or study integrity. Any participant taking over 120mg/day of prescribed methadone. Current severe DSM-5 mental health disorder (excluding opiate dependence). Current moderate or mild DSM-5 depressive, anxiety, sleep or personality disorders will be allowed given the high levels of comorbidity, provided in the opinion of the investigators, the participant is able to complete study procedures satisfactorily. Current or past history of enduring severe mental illness e.g. psychotic disorder (excluding drug induced), schizophrenia, bipolar affective disorder). Active suicidality. Use of regular prescription medications which in the opinion of the investigators will interfere with subject safety or study integrity. Regular use of psychotropic medication will be permitted e.g. antidepressants, provided the participant is compliant with administration and the investigators concur that they will not interfere with subject safety or study integrity. Participants are taking any medication that is contraindicated with baclofen or placebo (vitamin D3), or are hypersensitive to them or any of their excipients. Participants that are taking any medication that in the opinion of the investigators may impact on the outcome measures during the experimental session. Use of intermittent psychotropic medication which in the opinion of the investigators will interfere with subject safety or study integrity. End stage or acute renal failure. Severe chronic obstructive pulmonary disease (COPD) or Type 2 respiratory failure. Pulse rate <40 or >100 BPM OR systolic blood pressure >160 and <100 and a diastolic blood pressure >95 and <60 in the semi-supine position. Oxygen saturation <92% at rest A screening ECG with a QTcB or QTcF > 500 msec or an ECG that is not suitable for QT measurements (e.g. poorly defined termination of the T-wave) and/or with another ECG abnormality which in the opinion of the study physician is clinically significant and represents a safety risk. Note that if the initial QTc value is prolonged, the ECG should be repeated two more times (with 5 minutes between ECG readings) and the average of the 3 QTc values used to determine eligibility. Clinically significant head injury (e.g., requiring medical or surgical intervention) that in the opinion of the investigators, contraindicates their participation . Active hepatitis or HIV. Active peptic ulceration. Significant current or past medical history that, in the opinion of the investigators, contraindicates their participation. The subject has participated in a clinical trial and has received an investigational product within 30 days, 5 half-lives or twice the duration of the biological effect of the investigational product (whichever is longer) prior to the first experimental visit. Pregnancy or breast-feeding Unwillingness or inability to follow the procedures outlined in the protocol.