I took 50 mg a day (20 in the morning, and then 3 times 10 mg throughout the day.) It was prescribed for POTS and IST, but mainly helped the latter. I still had a big jump in hr when standing up. It also made my lower blood pressure even lower, which did not help my OI. But at least my hr wasn’t randomly 150 anymore.

It’s not always fun but it helps me prevent PEM as much as possible. Though sometimes there is no escaping it. Especially when you are more severe. I recommend Classic Pacing For A Better Life With ME by Ingeborg Dahl for tips. A HR monitor (eg Garmin, fitbit, visible) also helps!