As well as being an antiandrogen, spironolactone has a diuretic effect and also relaxes the blood vessels in the body. This results in decreased blood pressure.
If you are already taking antihypertensive medication to lower your blood pressure, spironolactone may further decrease your blood pressure. This could result in hypotension, which is where the blood pressure decreases to a point where sufficient blood flow to the organs cannot be maintained. This may result in symptoms such as fainting, headache, dizziness, confusion, fatigue, and decreased urine output.
For this reason, if you are taking antihypertensive medication to lower your blood pressure, then spironolactone is not recommended as an antiandrogen. Instead, it is recommended that you consider an alternative antiandrogen that does not affect blood pressure. These include gonadotropin releasing hormone analogues (nafarelin, triptorelin, leuprorelin), 5-alpha-reductase inhibitors (finasteride, dutasteride), and cyproterone acetate.