The use (abuse) of drugs and/or alcohol is quite common among those that suffer from PTSD and prior usage doesn't matter at all. The drugs and/or alcohol can provide a temporary escape from the severe depression that PTSD creates. Personally, for example, I found the depression to occur mostly when I was at home alone, generally tired, and often before going to in bed. To avoid this I'd take speed (meth or other forms) so that I wasn't tired but instead in "hyper-mode" and it prevented the symptom of depression and when I finally "burnt out" I was so tired that I went to sleep immediately eliminating the timeframe when the depression set in. It would have made no difference whether I'd taken speed before enduring combat or not because the use was directly related to the avoidance of the PTSD symptom after the experience of combat.