Evaluating the Safety of Acute Baclofen in Methadone-maintained Individuals With Opiate Dependence.

Inclusion Criteria:

1. Male or female

2. Aged over 21

3. Willing and able to comply with protocol

4. Able to read, comprehend and record information written in English

5. Capable of giving written informed consent, which includes compliance with therequirements and restrictions listed in the consent form.

6. Healthy as determined by a responsible physician, based on a medical evaluationwhich includes medical history, a physical examination, laboratory tests (ifrequired), and a psychiatric evaluation. A volunteer with clinical parametersoutside the reference range for the population being studied may be included, onlyif the investigators concur that the finding is unlikely to jeopardize eithersubject safety or study integrity.

7. DSM-5 diagnosis of current severe opioid use disorder

8. Treated with methadone substitution therapy and able to maintain the same stabledose for screening and experimental visit.

9. Ability to receive an acute dose of up to 90mg baclofen or up to 4800IU vitamin D (placebo).

Exclusion Criteria:

1. Intoxication on any of the visits, as assessed by difficulty in walking, theslurring of speech, difficulty concentrating or drowsiness. This exclusion criteriawould exclude a subject from that study day only and not the whole study, at thediscretion of the research team.

2. Positive urine drug screens or breath alcohol at screening or experimental testingvisits. A minimum list of drugs that will be screened for include amphetamines,cocaine, opiates, methadone, cannabinoids and benzodiazepines. Positive results formethadone will be allowed for those opiate dependent participants still undergoingOST. Positive results for cannabinoids will be allowed given the long half-life ofcannabinoid metabolites. This exclusion criteria would exclude a subject from thatstudy day only and not the whole study, at the discretion of the research team.

3. Current DSM-5 substance dependence disorder for any other substance except foropiates and nicotine. Lifetime history of dependence on other substances will beallowed given very high incidence of co-dependence.

4. Regular on-top use of heroin or other opiates or other illicit substances incombination with OST, which in the opinion of the investigators will interfere withsubject safety or study integrity.

5. Any participant taking over 120mg/day of prescribed methadone.

6. Current severe DSM-5 mental health disorder (excluding opiate dependence). Currentmoderate or mild DSM-5 depressive, anxiety, sleep or personality disorders will beallowed given the high levels of comorbidity, provided in the opinion of theinvestigators, the participant is able to complete study procedures satisfactorily.

7. Current or past history of enduring severe mental illness e.g. psychotic disorder (excluding drug induced), schizophrenia, bipolar affective disorder).

8. Active suicidality.

9. Use of regular prescription medications which in the opinion of the investigatorswill interfere with subject safety or study integrity. Regular use of psychotropicmedication will be permitted e.g. antidepressants, provided the participant iscompliant with administration and the investigators concur that they will notinterfere with subject safety or study integrity.

10. Participants are taking any medication that is contraindicated with baclofen orplacebo (vitamin D3), or are hypersensitive to them or any of their excipients.

11. Participants that are taking any medication that in the opinion of the investigatorsmay impact on the outcome measures during the experimental session.

12. Use of intermittent psychotropic medication which in the opinion of theinvestigators will interfere with subject safety or study integrity.

13. End stage or acute renal failure.

14. Severe chronic obstructive pulmonary disease (COPD) or Type 2 respiratory failure.

15. Pulse rate <40 or >100 BPM OR systolic blood pressure >160 and <100 and a diastolicblood pressure >95 and <60 in the semi-supine position.

16. Oxygen saturation <92% at rest

17. A screening ECG with a QTcB or QTcF > 500 msec or an ECG that is not suitable for QTmeasurements (e.g. poorly defined termination of the T-wave) and/or with another ECGabnormality which in the opinion of the study physician is clinically significantand represents a safety risk. Note that if the initial QTc value is prolonged, theECG should be repeated two more times (with 5 minutes between ECG readings) and theaverage of the 3 QTc values used to determine eligibility.

18. Clinically significant head injury (e.g., requiring medical or surgicalintervention) that in the opinion of the investigators, contraindicates theirparticipation .

19. Active hepatitis or HIV.

20. Active peptic ulceration.

21. Significant current or past medical history that, in the opinion of theinvestigators, contraindicates their participation.

22. The subject has participated in a clinical trial and has received an investigationalproduct within 30 days, 5 half-lives or twice the duration of the biological effectof the investigational product (whichever is longer) prior to the first experimentalvisit.

23. Pregnancy or breast-feeding

24. Unwillingness or inability to follow the procedures outlined in the protocol.