Because corticosteroids lower the resistance to infection, patients on steroid therapy cannot be vaccinated for smallpox or immunized. The administration of corticosteroids also causes underproduction of the natural hormones by the adrenal cortex, and so ACTH or corticosteroid therapy must always be withdrawn gradually. In addition, when used in large doses for long periods of time, the drugs can cause atrophy of the adrenal cortex. Side effects of steroid therapy include glaucoma, excess hair growth, and imbalance of many substances, including calcium, nitrogen, potassium, and sodium. Many of the synthetic corticosteroids, such as prednisone, prednisolone, triamcinolone, and betamethasone, are more potent than the naturally occurring compounds.