In chronic users, their body's natural ability to produce glucocorticoids are suppressed, whether it's due to a disease or to the chronic use of steroids themselves. However in a stress-state, like surgery or infection, the body normally produces extra cortisol and ACTH to prepare the body for the increased metabolic and circulatory demand associated with these states. In chronic users, their adrenal's are unable to ramp up this additional production. Studies have shown that stress-dosing additional glucocorticoids in these patients results in better outcomes.
because chronic users have adrenal suppression so they need a higher dose to deal with stress. infection - lower bp due to response to infection from cytokines and other inflammatory products, need more catecholamines to maintain map etc.