Methadone is a member of the class of drugs known as opioids. It is used for treatment of chronic pain and as a replacement therapy for those addicted to more dangerous opioid drugs, such as heroin. Xanax belongs to the class of benzodiazepines and is used mainly for anxiety. Methadone and Xanax taken together can have potentially life-threatening effects in a person who has never taken opioids, but these problems are unlikely in a long-term user. Abuse it the most serious long-term concern of Xanax use in patients on methadone replacement therapy.
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One effect of methadone and other opioids is a decrease in the respiratory drive originating from the brain. This drive can be diminished to the point where breathing stops entirely, which is what causes death in the case of an opiod overdose. Benzodiazepines also cause respiratory depression, although through a different mechanism than opioids. When the respiratory depression of opioids and benzodiazepines are combined, the effect is multiplied and the the risk of dangerously decreased levels of breathing is greatly increased, according to "Principles of Pharmacology." Tolerance to many of the effects of opioids, and to a lesser extent those of benzodiazepines, develops quickly, however, and respiratory depression is particularly notable in this respect. Long-term users of opioids can safely tolerate a dose many times greater than that which would be fatal to someone taking them for the first time. Since methadone is used almost exclusively as a long-term treatment for addiction or pain, the combination with Xanax is less likely to cause a risk of serious respiratory depression unless the dose of either drug is suddenly increased significantly for some reason.
Sedation is another effect that methadone and benzodiazepines have in common. "Principles of Pharmacology" says that when methadone and a benzodiazepine are combined, the sedative effects are multiplied. While sedation is not necessarily dangerous in and of itself, it often occurs in tandem with respiratory depression and makes it significantly more dangerous. Patients with impaired breathing who are unconscious or semi-conscious are much more likely to experience problems such as choking or aspirating vomit into the lungs. Such a degree of sedation is unlikely, however, in a long-term methadone user who is also taking Xanax. Tolerance to the sedative effects of methadone tends to develop as rapidly as tolerance to the respiratory depression, and long-term users of stable doses of methadone experience little, if any, sedative effect from it.
Benzodiazepines in general have a high potential for abuse, according to "American Family Physician," and Xanax is particularly bad in this respect due to its rapid onset and short duration of action. Patients who have had previous problems with substance abuse are at much greater risk for abuse of drugs such as Xanax, even if they are originally used for legitimate medical reasons. By definition, patients on methadone maintenance therapy have had prior substance abuse issues, benzodiazepines pose significant risk of abuse for them. Given that there are much better choices for the long-term treatment of anxiety, such as anti-depressants and behavioral approaches, benzodiazepines have very limited legitimate use for patients on methadone maintenance therapy. They might be appropriate for extremely short-term treatment of highly-acute anxiety disorders, such as panic attacks, until the long-term treatments start to become effective, but they should be phased out after a few weeks and avoided entirely if at all possible, according to "Harrison’s Principles of Internal Medicine."