The aims of this study are to 1) explore the efficacy of hyperbaric oxygen therapy (HBOT) applied at two different pressures compared to sham treatment in relieving signs and symptoms (both physical and psychological) of opioid withdrawal in human subjects associated with methadone dose reductions; and 2): explore whether HBOT can increase the odds of successful methadone dose reduction in patients who are interested in tapering their opioid dose. The investigators aim to explore, through qualitative methods, individual's experiences with treatment for opioid use disorder (OUD).

The investigators will conduct a prospective, randomized, sham treatment-controlled trial to examine the efficacy of HBOT for improving signs and symptoms of opioid withdrawal in methadone-maintained OUD patients as they undergo two planned methadone dose reductions spaced two weeks apart.

A participant sample size of 24 was chosen based on power analysis guided by the investigators' earlier study that indicate that clinically meaningful results can be found with a minimum of 8 per group related to variables of highest interest i.e., pain and withdrawal symptoms.

Additionally to this trial, participants will be asked to partake in a semi-structured interview in which they will be asked to discuss (1) Beginning Treatment, (2) Life in Recovery, and (3) HBOT & Further Treatment.

Edited 11/23/24: An additional study arm was added to better address the question about the level of HBOT needed for effects. The new participant sample size of 36 was chosen based on power analysis guided by our earlier study that indicate that clinically meaningful results can be found with a minimum of 8 per group related to variables of highest interest i.e., pain and withdrawal symptoms. Participants will be randomized into one of three trial arms: (active arm of n = 12 in HBOT 100% oxygen and 2.0 ATA, partial active arm of n = 12 in HBOT 100% oxygen and 1.3 ATA, and sham arm of n = 12 in HBOT 21% oxygen and 1.3 ATA). All groups will be receiving standard of care for addiction withdrawal symptoms, as managed and provided by their clinic physician and neither the full or sub-therapeutic HBOT dose are expected to affect their ongoing treatment.

Inclusion Criteria:

• Enrollment in a supervised opioid treatment program at the Spokane Regional Health District
• Age greater than 18 years
• Ability to read, speak, and write English
• Ability to provide written informed consent.

Exclusion Criteria:

• Pregnancy
• Any other medical or psychiatric condition that the PI, Co-PI (physician of record) or the Spokane Hyperbaric Center medical director determine might compromise safe study participation in the HBOT clinic (including but not limited to active psychosis, history of frequent psychiatric hospitalizations, severe anxiety with claustrophobia, aggression)
• Upper respiratory infection
• Emphysema
• Air cysts in the lung
• History of thoracic or ear surgery
• Taking the medication Antabuse for alcohol addiction
• High fever.

Only the HBOT personnel will be aware of the treatment assignments; the rest of the research team and participants will be blinded to condition.