Does anyone know if the use of a diuretic for hbp causes impotence? My husband currently takes an ACE inhibitor, and his doc wants to add the diurectic.

I think ALL BP meds AND BP itself can cause impotence over a long enough time.
We just have to weigh risks, often with incomplete information :(.

Yes, diuretics are implicated, thiazides more than loop diuretics (like LASIX <furosemide.>)

Some theory is that ACE inhibitors and ARBs don't have this effect but my jury is most definitely out on it til some decent studies come in...and it's hard to study anything that takes decades to develop data.

How does a diuretic lower bp? What is the mechanism?

My opinion is that all bp medicines have the capacity to cause impotence, although it is hard to tell the source of any problem someone might have in that area. Trial-and-error is the only method, and weighing the costs and benefits is what your'e stuck with in the case of any powerful medicine.