The drug itself contains potassium although pretty small amounts, not sure if thats why the warning or if it somehow affects potassium uptake.
The drug itself scares the hell out of me and if your only on it for blood pressure I would be chasing nutritional aid in lowering BP instead of drugs. High BP if often a sign of one or more deficiencies. One of the biggest that comes to mind is vit C. Lack of it causes arterial stiffening and blockage. Read up on pauling therapy.