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Steroid agonists are substances that act by binding to the same steroid receptors on cells as naturally occurring steroids, producing similar effects in the body. They can enhance or stimulate the receptor's response, mimicking the effects of the endogenous steroids. On the other hand, steroid antagonists (or also known as competitive antagonists) are substances that bind to the same steroid receptors on cells but don't activate the receptors or produce any of the desired effects. Instead, they block the receptor from binding with the natural steroids, preventing the activation of the receptor and thus inhibiting the effects of the endogenous steroids.

A specific example of a drug that functions as a steroid agonist is prednisone. Prednisone is a synthetic corticosteroid drug that acts on the same glucocorticoid receptors as cortisol, which is a naturally occurring hormone. Prednisone is used to treat a wide range of diseases and conditions, including autoimmune disorders, asthma, and inflammation, by suppressing the immune system and reducing inflammation in the body.

A specific example of a drug that functions as a steroid antagonist is mifepristone (RU-486). Mifepristone is a progesterone receptor antagonist which competes with progesterone for binding to its receptor. This drug is used for medical termination of pregnancy, as it blocks the effects of progesterone, a hormone necessary for the maintenance of pregnancy. By blocking progesterone, mifepristone causes the breakdown of the endometrium, leading to the detachment of the embryo and uterine contractions that help expel the contents of the uterus. In summary, the main difference between steroid agonists and antagonists is that agonists mimic the effects of natural steroids, activating the receptors in the body, whereas antagonists block the receptors from binding with natural steroids, preventing the activation of the receptors and inhibiting the natural effects of the steroids. Prednisone (a glucocorticoid receptor agonist) and mifepristone (a progesterone receptor antagonist) are specific examples of each type of drug.