If you don't need it, don't take it. If you think that you might need it, consult your doctor, as there may be underlying causes for your concerns. If you don't have diabetes, high or low blood pressure, or take other meds that may effect your "ability", simply changing your diet and exercise habits can do wonders. Other things can have an adverse effect as well, such as alcohol, tobacco, and caffeine.

Be smart and careful. Don't take drugs that you don't need.