hctz is a diuetic that can lower your blood pressure. it should not decrease your stamina but it could be related to your light-headedness with standing quickly (orthostatic hypotension). i would suggest that you see your medical provider -- the only that prescribed the hctz -- for a consultation.
i am an avid runner but since starting 25 mg of hctz my stamina and speed has dropped off greatly. is this correlated? . i typically run 3 miles a day but am now winded and light-headed after 1 mile. my times have dropped markedly. i've also noticed that i get very light-headed whenever i stand up quickly and that is also new since beginning this drug. my blood pressure is on the high side of normal.