Have you ever realised why he became an alcoholic and drug addict? When he was on Buffy he started having panic attacks and decided to a gym and went for a while. Joss was absolutely furious with him Xander was not allowed to be fit and toned and gave him an ultimatum. Quit going to the gym or be fired. For a lot of actors being a main cast member of a show is like hitting a jackpot. Regular income and not having to go to auditions for guest spots and I can totally understand why he agreed to stop going to the gym. Without that outlet for his mental health he started drinking heavily which is why in later seasons he is overweight and flabby, same as Chandler was when Matthew Perry was drinking a lot. Then Nicky got injured and he was prescribed opioids.