How many missed doses of hypertension (high blood pressure) medication will be dangerous for the elderly 85 years old and older, that potentially causing a stroke?

It depends on high the blood pressure is without the medicine. A doctor who knows the patient can evaluate this. If they are supposed to take the medicine for blood pressure, they should not stop it unless that is the advice of their doctor. If their blood pressure is too low, it may be right to stop it. Some blood pressure meds should be weaned off so their doctor should advise. Good luck!

No clue about the number of "missed doses" which cause a stroke. But chronic hypertension is a prime stroke risk. If the 85 yo is missing medication on a regular basis, one should consider memory loss issues and dementia. All of this needs to be addressed by your primary care physician ASAP.