Grayanotoxin is a neurotoxin with a lot of unpleasant side effects. It would make a terrible drug for high blood pressure.

because we already have actual medication that does that in a controllable and well-characterized way

There could be a case for doing it for people that will refuse anything but natural if it doesn't have other problems, assuming not synthesized, but there's a problem of funding the safety studies and standardization process if a patent is unlikely to result.