Doctor wanted to use 25mg spironolactone to counter effect potassium lose by hct in ben/hct. I wonder why not use Benicar (olmesartan) without hctz. How effective.?

A good option. Benicar (olmesartan) ; hctz work so well together that the combination tablet was formulated. Potassium levels are often sustained with this combination but some folks lose too much potassium even with this low dose of hctz. An alternative to spironolactone is potassium but it can irritate the stomach. Also spironolactone itself can help lower blood pressure.
Spironolactone. Can reverse the loss of potassium due to hctz. Hctz is used to increase the potency of blood pressure lowering with the benicar, (olmesartan) which can also be achieved by increasing the dose of Benicar (olmesartan) without hctz, if possible. I do not use hctz at all, as most of my patients have diabetes, and hctz can increase Insulin resistance, blood sugar, and lipids. Talk w/your doctor.
Potassium loss? Spironolactone (s) can increase potassium (k) levels. K loss by diuretics (d) can be caused by 2 factors. D's waste both k and magnesium (m) in the urine. If a patient has a low m, the k will not be corrected until the m level is restored to normal. S cannot increase the m level at all. Using Benicar (olmesartan) with no diuretic may not be enough to reduce your BP to goal. Discuss this with your physician.