A agonist of alpha and beta receptores would be something like noradrenaline or epinephrine.
A agonist like that would cause the following side effects associated with the heart : vasoconstriction and the work of the heart increases. If the patient suffers from coronary heart disease, it would also cause angina, from the oxygen demand. Some other side effects would be sedation, impotence, bradycardia, orthostatic hypotension, and impotence, constipation, nausea, etc.