The therapeutic efficacy  of antihypertensive drugs is blunted by NSAIDs because they:
PGE2 and PGI2 cause renal vasodilation, natriuresis and increased water clearance due to inhibition of the action of ADH.
Loop diuretics act partly by increasing the stimulation of COX; therefore NSAIDs attenuate the diuretic action of these drugs.
Cause sodium excretion
Increase the clearance of antihypertensive drugs
Decrease the absorption of antihypertensive drugs
Decrease the synthesis of vascular prostacyclin