Metoprolol is a beta blocker, and as a previous poster said it's used for hypertension (sometimes other reasons too, but hypertension is the major one). These drugs work by blocking receptors for epinephrine (adrenaline), mainly in the heart, and this reduces the rate and force with which the heart contracts so yes, it does reduce cardiac output. One of the side effects can be reduced exercise capacity because the drug keeps the heart rate from going as high during exertion, and some people say beta blockers make them feel a bit tired in general (especially the ones like propranolol that are less specific for the heart and block epinephrine receptors everywhere else too). A couple of the other common side effects can be increased sensitivity to cold and feeling a bit depressed. They are also not usually recommended for people with asthma because blockade of the receptors in the airways can cause them to constrict more. All that said, the beta blockers are generally considered a pretty safe class of drugs. Hope this info helps!