How low does someone's blood pressure have to be to hold a blood pressure medication?

It depends.: A blood pressure is too low if a person feels light-headed or faint. For many patients a blood pressure below 100 systolic (the upper number in the blood pressure reading) is too low. However some patients with congestive heart failure may do better with a lower reading. A rapid drop in blood pressure may cause symptoms even though the reading may not be very low.

Symptoms, not number: A more practical way to think of this is to use symptoms, not numbers. If you often are dizzy after taking your BP med, your BP is too low for you. Ideally, BP should be kept <135 but some people can't tolerate that BP if they're used to higher numbers. The lower, the better. So if your BP is 100/60 and you feel great, keep taking your meds.