Updated November 28, 2014.

Naltrexone is an opioid receptor antagonist used primarily in the management of alcohol dependence and opioid addiction. Naltrexone hydrochloride is sold as the brand name Revia and Depade. An extended-release form of Naltrexone is marketed under the trade name Vivitrol.

How Does Naltrexone Work?

For people who have stopped drinking, Naltrexone reduces the craving for alcohol which many alcohol dependent people experience when they quit drinking.

It is not fully understood how Naltrexone works to reduce the craving for alcohol, but some scientists believe it works by affecting the neural pathways in the brain where the neurotransmitter dopamine is found.
For those who are addicted to opioids, Naltrexone works by blocking the effects of drugs like heroin and cocaine in the brain. As an opioid receptor antagonist, Naltrexone simply blocks the normal reaction of the part of the brain that produces the feeling of pleasure when opioids are taken.

How Is Naltrexone Taken?

In pill form, Naltrexone is usually prescribed to be taken once a day. Generally, it is prescribed for 12 weeks to help people who have stopped drinking to reduce the craving for alcohol during the early days of abstinence when the risk of a relapse is the greatest. Because Naltrexone blocks the effects of opioids, it is sometimes prescribed for 12 months for those trying to manage drug dependence.
In April 2006, the FDA approved an once-a-month injectible form of Naltrexone, which is marketed as Vivitrol, for the treatment of alcohol dependence.

Several studies demonstrated the monthly injection form of Naltrexone was more effective in maintaining abstinence over the pill form, because it eliminates the problem of medication compliance.

Rapid Detoxification

An implant form of Naltrexone is used in a controversial process called rapid detoxification for opioid dependence. In rapid detox, the patient is placed under general anaesthesia and a Naltrexone implant is surgically placed in the lower abdomen or posterior. This procedure is usually followed by daily doses of Naltrexone for up to 12 months.
The FDA has not approved the implant form of Naltrexone. Although the rapid detox procedure is promoted as a one-time "cure" for drug addiction, research has shown that it is really more effective as an initial step in a long-term rehabilitation process.

What Are the Side Effects?

For some patients, Naltrexone can cause upset stomach, nervousness, anxiety or muscle and joint pain. Usually these symptoms are mild and temporary, but for some they can be more severe and longer lasting.
In rare cases, Naltrexone can cause more severe side effects including confusion, drowsiness, hallucinations, vomiting, stomach pain, skin rash, diarrhea or blurred vision. Anyone experiencing any of these symptoms should notify their healthcare provider immediately.

Large doses of naltrexone can cause liver failure. Patients should stop taking Naltrexone immediately if they experience any of the following symptoms: excessive tiredness, unusual bleeding or bruising, loss of appetite, pain in the upper right part of the stomach, dark urine, or yellowing of the skin or eyes, according to the warning published on the Naltrexone packaging.

Who Can Take Naltrexone?

Naltrexone is prescribed only after the person has stopped drinking alcohol or taking opioids for seven to 10 days, because it can cause serious withdrawal symptoms if it is taken while someone is still using drugs.
People who have acute hepatitis, liver or kidney disease should not take Naltrexone. Patients who are using narcotic painkillers should not take it nor should anyone who is allergic to any other drugs. Women who are pregnant or breastfeeding, should not take Naltrexone.

What Naltrexone Does Not Do

Naltrexone does not help someone stop drinking or doing drugs, it is used to help people who have already stopped maintain abstinence. It does not treat alcohol or drug withdrawal symptoms.

How Effective Is Naltrexone?

Research has shown that Naltrexone can reduce craving for alcohol and drugs for some people, but it does not work for everyone. Like most pharmaceutical treatments for alcohol and drug abuse, it works best if it is used in connection with an overall treatment regime, such as psychosocial therapy, counseling or support group participation.
Naltrexone does not "cure" addiction, but it has helped many who suffer from alcohol or drug addiction to maintain abstinence by reducing their craving for alcohol or drugs.

Sources:
U.S. National Library of Medicine
National Clearinghouse for Alcohol and Drug Information
National Institute on Alcohol Abuse and Alcoholism