Yes it is very bad. In fact, I have normal blood pressure and was accidentally given high blood pressure medication (somehow they mixed it up with generic tylenol) and I about fainted. Chills and dizziness for a couple of hours. So definitely do NOT share blood pressure meds.
You shouldn't share prescription meds anyways - not only is it illegal, but since you are not their doctor you don't know what kinds of problems that medication can cause for them.