NaltrexoneA very important drug in medicine, employed to the treatment of alcohol and opium-based drug dependency and abuse. Naltrexone is a generic drug, but when looking at branded solutions you can find it under the name Vivitrol. 
A lot of people develop a dependency on pain medication such as Oxycodone and Codeine. These are the situations where Naltrexone is being introduced in the treatment, so as to avoid the development of said dependency.
There are analogues of this medicine that are created to help people with various types of addiction and other impulse related disorders, such as autism, compulsive gambling and tobacco dependency.
Generally, Naltrexone is a single part of a larger treatment when dealing with specific dependencies and is often implemented alongside other non-drug related methods. People suffering from addiction are supported to change their lifestyle and have consulting available to them for the duration of the treatment.
Naltrexone has been approved by the FDA in the early 2010's because it is safe for use and  the need for a drug like this on the market is evident. 
The specific way that it works is as an inhibitor, which disables the effects of addictive drugs. 
There are also many off-label uses for this drug, mainly to reduce pain related issues and other symptom based illnesses, although this isn't recommended.