Medication-assisted treatment, used together with counseling and behavioral therapy, is known to be effective in people who have opioid use disorder. There are three drugs approved by the Food and Drug Administration (FDA) to treat opioid use disorder: buprenorphine, methadone, and naltrexone. Each comes in various forms or preparations, and each variation has a unique trade name. In light of the current opioid abuse epidemic in the United States, the FDA is encouraging that medication-assisted treatment be offered to all patients addicted to opioids. Patients should be active partners in selecting the specific drug that would be best for them. Nurses may begin to see these drugs prescribed more frequently, and patients are likely to ask nurses to help them understand the different treatment options. The following is a brief overview of the mechanism of action of each of these three medication-assisted treatments.

Buprenorphine is a partial agonist at the mu opioid receptor and an antagonist at the kappa opioid receptor. The drug has high affinity for opioid receptors on nerve cells in the brain and spinal cord. Stimulation of mu opioid receptors acts directly on the brainstem respiratory centers and may cause respiratory depression. Use of buprenorphine produces constant miosis, and tolerance does not develop to this effect. As a partial agonist at the mu receptor, buprenorphine displaces full agonists such as morphine and methadone. Buprenorphine's blockade of kappa opioid receptors helps to control drug-seeking behavior and drug cravings, as these receptors are found in areas of the brain associated with motivation, emotion, and cognition.

Methadone is a mu opioid agonist and acts similarly to morphine. Like buprenorphine, methadone produces respiratory depression and constant miosis. Both drugs have a long half-life, which accounts for their usefulness in treating opioid dependence: neither causes the swings in drug plasma levels that can lead to overdose or to the withdrawal symptoms associated with short-term opioids like heroin. Unlike buprenorphine, methadone does not control drug-seeking behavior or cravings. Methadone may be the most familiar of the medication-assisted treatments to nurses since it was used to treat opiate addiction starting in the mid-1960s and was FDA approved for this use in 1972.

Naltrexone is an antagonist with affinity for all opioid receptors. Because it is highly competitive for the receptor sites and almost completely displaces opioids or blocks their effects, it induces immediate withdrawal symptoms in patients who are still physically dependent on opioids. Naltrexone has a weaker effect on the third opioid receptor, delta, than on the mu and kappa opioid receptors. Delta opioid receptors are believed to act on the amygdala, hippocampus, and prefrontal cortex to enhance mood. They play a role in developing tolerance to morphine, and are also involved in the impulse to seek out opioids and in the development of psychological dependence on opioids, especially morphine. Naltrexone blocks these actions of delta opioid receptors.

Naltrexone is different from the drug naloxone, which also blocks opioid receptors. Naloxone is used in overdose emergencies, as its onset is rapid and its duration of action is short. Naltrexone, which is long acting, is used in long-term treatment of opioid abuse, starting after the patient has fully eliminated (detoxified) the opioid from her or his system.

For more information and a list of FDA-approved buprenorphine, methadone, and naltrexone products, see http://www.fda.gov/Drugs/DrugSafety/InformationbyDrugClass/ucm600092.htm.