If you're already taking medications to lower your blood pressure then you should avoid drinking too much Hibiscus Tea since if you do drink too much Hibiscus Tea while taking blood pressure medicines it can cause your blood pressure to get too low.
The resulting blood pressure that goes too low could get dangerously low and if you do notice your blood pressure getting too low while drinking Hibiscus Tea then stop drinking the Hibiscus Tea and let your doctor know about it.