Relapse happens. Relapse is extremely painful and can cause a great deal of hurt for everyone involved. Relapse is defined when an individual goes back to using substances after not using for a significant period of time. This definition can vary depending on who you talk to.
There are signs to look for that may lead to a relapse (see http://www.liverehab.com/relapse-warning-signs/) . Once someone is triggered an overwhelming urge will come over and if that individual has not been taught how to deal with triggers then it is likely that they will unconsciously use again. Basically, there is a circuit in the brain that can be activated in an amount of time that is significantly short which makes it impossible for the individual to be aware that it is even happening.
Everybody’s brain is wired differently and that is why some people have a problem with addiction and others do not. Those who struggle are likely to have a lifetime of struggle and that is why getting professional help is crucial to avoiding relapse.