Hi Siran, 12.5 mg of Metoprolol is a very low dose. You don t state the reason why your mother was on the drug. Has she had a heart attack,? does she have congestive heart failure? What other medications is she taking? Did she have any diagnostic tests done before she saw the Dr, like an echocardiogram or a chest x-ray? Metoprolol can weaken the force of the hearts contraction, which is what I think her Dr.was trying to explain to her. While this would decrease the hearts oxygen demands when it does not pump blood as effectively it can lead to congestive heart failure. In medical terms this is called a negative inotropic effect. This is only speculation on my part based on the information you have provided in your question, you can email be if you need to, there s nothing worse than a sick mom, take care, Donna