PTSD and alcohol use and/or drug use problems frequently co-occur. People with PTSD are more likely than people without PTSD to have problems with drugs and/or alcohol. There is currently a lot of debate about how best to treat PTSD and substance use problems. Many mental health professionals believe that for people with PTSD and a co-occurring substance abuse problem, you have to treat the substance use problem first. Substance use can interfere with traditional treatments for PTSD, such as exposure therapy, or some medications.
However, while substance use can definitely interfere with therapy, people may be using substances as a way of managing the distressing symptoms of PTSD. Therefore, if you take away the substance use without helping them learn other ways of coping with their symptoms, they may quickly go back to abusing substances.
For this reason, there is a movement to develop treatments that address both PTSD and substance use problems concurrently, with the idea that they are very much intertwined. A good example of one of these treatments is Seeking Safety, developed by Dr. Lisa Najavits.