On 15 October, 1918, Adolf Hitler himself fell victim to mustard gas attack on the battlefield south of Wervick, part of the southern front near Ypres in Belgium. Interestingly enough, the mustard gas was invented and first used as a chemical weapon by the Germans in 1916 but subsequently adopted by the Entente powers as well.
Hitler and his comrades were temporarily blinded by the gas (Hitler also temporarily lost his voice) and found their way to safety only by clinging on to each other and following a comrade who was slightly less badly afflicted.
It took Hitler over a month to recover from the short-term effects of mustard gas poisoning. The long-term effects, however, were a different matter entirely. Especially the effects on the mental health of the poisoned individual.
Studies have shown that individuals poisoned with mustard gas were suffering from a number of psychological symptoms even twenty years after exposure. These symptoms include increased hostility, anxiety, obsessive-compulsive syndrome, depression and the general post-traumatic stress disorder.
From what we know about the post-war decisions and actions (and the general behavior) of Adolf Hitler, it can be concluded that he most likely suffered from “all the above” for the rest of his life.